index edit block technology

ch02.asciidoc

@@ -234,7 +234,7 @@ If Bob's bitcoin wallet application is directly connected to Alice's wallet appl
 
 === Bitcoin Mining
 
-((("mining and consensus", "overview of", id="MACover02")))((("blockchain technology", "overview of mining", id="BToverview02")))Alice's transaction is now propagated on the bitcoin network. It does not become part of the _blockchain_ until it is verified and included in a block by a process called _mining_. See <<mining>> for a detailed explanation. 
+((("mining and consensus", "overview of", id="MACover02")))((("blockchain (the)", "overview of mining", id="BToverview02")))Alice's transaction is now propagated on the bitcoin network. It does not become part of the _blockchain_ until it is verified and included in a block by a process called _mining_. See <<mining>> for a detailed explanation. 
 
 The bitcoin system of trust is based on computation. Transactions are bundled into _blocks_, which require an enormous amount of computation to prove, but only a small amount of computation to verify as proven. The mining process serves two purposes in bitcoin:
 
@@ -267,7 +267,7 @@ You can see the block that includes https://blockchain.info/block-height/277316[
 
 ((("confirmations", "role in transactions")))Approximately 19 minutes later, a new block, #277317, is mined by another miner. Because this new block is built on top of block #277316 that contained Alice's transaction, it added even more computation to the blockchain, thereby strengthening the trust in those transactions. Each block mined on top of the one containing the transaction counts as an additional confirmation for Alice's transaction. As the blocks pile on top of each other, it becomes exponentially harder to reverse the transaction, thereby making it more and more trusted by the network.
 
-((("genesis block")))((("blocks", "genesis block")))((("blockchain technology", "genesis block")))In the diagram in <<block-alice1>>,  we can see block #277316, which contains Alice's transaction. Below it are 277,316 blocks (including block #0), linked to each other in a chain of blocks (blockchain) all the way back to block #0, known as the _genesis block_. Over time, as the "height" in blocks increases, so does the computation difficulty for each block and the chain as a whole. The blocks mined after the one that contains Alice's transaction act as further assurance, as they pile on more computation in a longer and longer chain. By convention, any block with more than six confirmations is considered irrevocable, because it would require an immense amount of computation to invalidate and recalculate six blocks. We will examine the process of mining and the way it builds trust in more detail in <<mining>>.((("", startref="BToverview02")))((("", startref="MACover02")))
+((("genesis block")))((("blocks", "genesis block")))((("blockchain (the)", "genesis block")))In the diagram in <<block-alice1>>,  we can see block #277316, which contains Alice's transaction. Below it are 277,316 blocks (including block #0), linked to each other in a chain of blocks (blockchain) all the way back to block #0, known as the _genesis block_. Over time, as the "height" in blocks increases, so does the computation difficulty for each block and the chain as a whole. The blocks mined after the one that contains Alice's transaction act as further assurance, as they pile on more computation in a longer and longer chain. By convention, any block with more than six confirmations is considered irrevocable, because it would require an immense amount of computation to invalidate and recalculate six blocks. We will examine the process of mining and the way it builds trust in more detail in <<mining>>.((("", startref="BToverview02")))((("", startref="MACover02")))
 
 [[block-alice1]]
 .Alice's transaction included in block #277316

ch07.asciidoc

@@ -195,7 +195,7 @@ Note that because the redeem script is not presented to the network until you at
 [[op_return]]
 === Data Recording Output (RETURN)
 
-((("transactions", "advanced", "data recording output")))((("scripting", "data recording output")))((("RETURN operator")))((("data recording (nonpayment data)")))((("nonpayment data")))((("blockchain technology", "nonpayment data recording")))((("digital notary services")))Bitcoin's distributed and timestamped ledger, the blockchain, has potential uses far beyond payments. Many developers have tried to use the transaction scripting language to take advantage of the security and resilience of the system for applications such as digital notary services, stock certificates, and smart contracts. Early attempts to use bitcoin's script language for these purposes involved creating transaction outputs that recorded data on the blockchain; for example, to record a digital fingerprint of a file in such a way that anyone could establish proof-of-existence of that file on a specific date by reference to that transaction.
+((("transactions", "advanced", "data recording output")))((("scripting", "data recording output")))((("RETURN operator")))((("data recording (nonpayment data)")))((("nonpayment data")))((("blockchain (the)", "nonpayment data recording")))((("digital notary services")))Bitcoin's distributed and timestamped ledger, the blockchain, has potential uses far beyond payments. Many developers have tried to use the transaction scripting language to take advantage of the security and resilience of the system for applications such as digital notary services, stock certificates, and smart contracts. Early attempts to use bitcoin's script language for these purposes involved creating transaction outputs that recorded data on the blockchain; for example, to record a digital fingerprint of a file in such a way that anyone could establish proof-of-existence of that file on a specific date by reference to that transaction.
 
 ((("blockchain bloat")))((("bloat")))((("unspent transaction outputs (UTXO)")))((("UTXO sets")))The use of bitcoin's blockchain to store data unrelated to bitcoin payments is a controversial subject. Many developers consider such use abusive and want to discourage it. Others view it as a demonstration of the powerful capabilities of blockchain technology and want to encourage such experimentation. Those who object to the inclusion of nonpayment data argue that it causes "blockchain bloat," burdening those running full bitcoin nodes with carrying the cost of disk storage for data that the blockchain was not intended to carry. Moreover, such transactions create UTXO that cannot be spent, using the destination bitcoin address as a freeform 20-byte field. Because the address is used for data, it doesn't correspond to a private key and the resulting UTXO can _never_ be spent; it's a fake payment. These transactions that can never be spent are therefore never removed from the UTXO set and cause the size of the UTXO database to forever increase, or "bloat."
 

ch08.asciidoc

@@ -145,9 +145,9 @@ If there is no traffic on a connection, nodes will periodically send a message t
 
 === Full Nodes
 
-((("bitcoin network", "full nodes")))((("full-node clients")))((("blockchain technology", "full blockchain nodes")))Full nodes are nodes that maintain a full blockchain with all transactions. More accurately, they probably should be called "full blockchain nodes." In the early years of bitcoin, all nodes were full nodes and currently the Bitcoin Core client is a full blockchain node. In the past two years, however, new forms of bitcoin clients have been introduced that do not maintain a full blockchain but run as lightweight clients. We'll examine these in more detail in the next section.
+((("bitcoin network", "full nodes")))((("full-node clients")))((("blockchain (the)", "full blockchain nodes")))Full nodes are nodes that maintain a full blockchain with all transactions. More accurately, they probably should be called "full blockchain nodes." In the early years of bitcoin, all nodes were full nodes and currently the Bitcoin Core client is a full blockchain node. In the past two years, however, new forms of bitcoin clients have been introduced that do not maintain a full blockchain but run as lightweight clients. We'll examine these in more detail in the next section.
 
-((("blocks", "genesis block")))((("genesis block")))((("blockchain technology", "genesis block")))Full blockchain nodes maintain a complete and up-to-date copy of the bitcoin blockchain with all the transactions, which they independently build and verify, starting with the very first block (genesis block) and building up to the latest known block in the network. A full blockchain node can independently and authoritatively verify any transaction without recourse or reliance on any other node or source of information. The full blockchain node relies on the network to receive updates about new blocks of transactions, which it then verifies and incorporates into its local copy of the blockchain.
+((("blocks", "genesis block")))((("genesis block")))((("blockchain (the)", "genesis block")))Full blockchain nodes maintain a complete and up-to-date copy of the bitcoin blockchain with all the transactions, which they independently build and verify, starting with the very first block (genesis block) and building up to the latest known block in the network. A full blockchain node can independently and authoritatively verify any transaction without recourse or reliance on any other node or source of information. The full blockchain node relies on the network to receive updates about new blocks of transactions, which it then verifies and incorporates into its local copy of the blockchain.
 
 ((("bitcoin nodes", "full nodes")))Running a full blockchain node gives you the pure bitcoin experience: independent verification of all transactions without the need to rely on, or trust, any other systems. It's easy to tell if you're running a full node because it requires 20+ gigabytes of persistent storage (disk space) to store the full blockchain. If you need a lot of disk and it takes two to three days to sync to the network, you are running a full node. That is the price of complete independence and freedom from central authority.
 
@@ -157,7 +157,7 @@ If there is no traffic on a connection, nodes will periodically send a message t
 
 ((("bitcoin network", "syncing the blockchain")))The first thing a full node will do once it connects to peers is try to construct a complete blockchain. If it is a brand-new node and has no blockchain at all, it only knows one block, the genesis block, which is statically embedded in the client software. Starting with block #0 (the genesis block), the new node will have to download hundreds of thousands of blocks to synchronize with the network and reestablish the full blockchain.
 
-((("blockchain technology", "syncing the blockchain")))((("syncing")))The process of syncing the blockchain starts with the +version+ message, because that contains +BestHeight+, a node's current blockchain height (number of blocks). A node will see the +version+ messages from its peers, know how many blocks they each have, and be able to compare to how many blocks it has in its own blockchain. Peered nodes will exchange a +getblocks+ message that contains the hash (fingerprint) of the top block on their local blockchain. One of the peers will be able to identify the received hash as belonging to a block that is not at the top, but rather belongs to an older block, thus deducing that its own local blockchain is longer than its peer's.
+((("blockchain (the)", "syncing the blockchain")))((("syncing")))The process of syncing the blockchain starts with the +version+ message, because that contains +BestHeight+, a node's current blockchain height (number of blocks). A node will see the +version+ messages from its peers, know how many blocks they each have, and be able to compare to how many blocks it has in its own blockchain. Peered nodes will exchange a +getblocks+ message that contains the hash (fingerprint) of the top block on their local blockchain. One of the peers will be able to identify the received hash as belonging to a block that is not at the top, but rather belongs to an older block, thus deducing that its own local blockchain is longer than its peer's.
 
 The peer that has the longer blockchain has more blocks than the other node and can identify which blocks the other node needs in order to "catch up." It will identify the first 500 blocks to share and transmit their hashes using an +inv+ (inventory) message. The node missing these blocks will then retrieve them, by issuing a series of +getdata+ messages requesting the full block data and identifying the requested blocks using the hashes from the +inv+ message.
 

ch09.asciidoc

@@ -3,7 +3,7 @@
 
 === Introduction
 
-((("blockchain technology", "overview of")))The blockchain data structure is an ordered, back-linked list of blocks of transactions. The blockchain can be stored as a flat file, or in a simple database. The Bitcoin Core client stores the blockchain metadata using Google's LevelDB database. Blocks are linked "back," each referring to the previous block in the chain. ((("blocks", "block height")))The blockchain is often visualized as a vertical stack, with blocks layered on top of each other and the first block serving as the foundation of the stack. The visualization of blocks stacked on top of each other results in the use of terms such as "height" to refer to the distance from the first block, and "top" or "tip" to refer to the most recently added block.
+((("blockchain (the)", "overview of")))The blockchain data structure is an ordered, back-linked list of blocks of transactions. The blockchain can be stored as a flat file, or in a simple database. The Bitcoin Core client stores the blockchain metadata using Google's LevelDB database. Blocks are linked "back," each referring to the previous block in the chain. ((("blocks", "block height")))The blockchain is often visualized as a vertical stack, with blocks layered on top of each other and the first block serving as the foundation of the stack. The visualization of blocks stacked on top of each other results in the use of terms such as "height" to refer to the distance from the first block, and "top" or "tip" to refer to the most recently added block.
 
 ((("blocks", "block hash")))((("blocks", "genesis block")))((("blocks", "parent blocks")))((("genesis block")))((("parent blocks")))Each block within the blockchain is identified by a hash, generated using the SHA256 cryptographic hash algorithm on the header of the block. Each block also references a previous block, known as the _parent_ block, through the "previous block hash" field in the block header. In other words, each block contains the hash of its parent inside its own header. The sequence of hashes linking each block to its parent creates a chain going back all the way to the first block ever created, known as the _genesis block_.
 
@@ -15,7 +15,7 @@ One way to think about the blockchain is like layers in a geological formation,
 
 === Structure of a Block
 
-((("blocks", "structure of")))((("blockchain technology", "block structure")))A block is a container data structure that aggregates transactions for inclusion in the public ledger, the blockchain. The block is made of a header, containing metadata, followed by a long list of transactions that make up the bulk of its size. The block header is 80 bytes, whereas the average transaction is at least 250 bytes and the average block contains more than 500 transactions. A complete block, with all transactions, is therefore 1,000 times larger than the block header. <<block_structure1>> describes the structure of a block.
+((("blocks", "structure of")))((("blockchain (the)", "block structure")))A block is a container data structure that aggregates transactions for inclusion in the public ledger, the blockchain. The block is made of a header, containing metadata, followed by a long list of transactions that make up the bulk of its size. The block header is 80 bytes, whereas the average transaction is at least 250 bytes and the average block contains more than 500 transactions. A complete block, with all transactions, is therefore 1,000 times larger than the block header. <<block_structure1>> describes the structure of a block.
 
 [[block_structure1]]
 [role="pagebreak-before"]
@@ -32,7 +32,7 @@ One way to think about the blockchain is like layers in a geological formation,
 [[block_header]]
 === Block Header
 
-((("blocks", "headers")))((("blockchain technology", "block headers")))((("headers")))The block header consists of three sets of block metadata. First, there is a reference to a previous block hash, which connects this block to the previous block in the blockchain. The second set of metadata, namely the _difficulty_, _timestamp_, and _nonce_, relate to the mining competition, as detailed in <<mining>>. The third piece of metadata is the merkle tree root, a data structure used to efficiently summarize all the transactions in the block. <<block_header_structure_ch09>> describes the structure of a block header.
+((("blocks", "headers")))((("blockchain (the)", "block headers")))((("headers")))The block header consists of three sets of block metadata. First, there is a reference to a previous block hash, which connects this block to the previous block in the blockchain. The second set of metadata, namely the _difficulty_, _timestamp_, and _nonce_, relate to the mining competition, as detailed in <<mining>>. The third piece of metadata is the merkle tree root, a data structure used to efficiently summarize all the transactions in the block. <<block_header_structure_ch09>> describes the structure of a block header.
 
 
 [[block_header_structure_ch09]]
@@ -53,7 +53,7 @@ The nonce, difficulty target, and timestamp are used in the mining process and w
 [[block_hash]]
 === Block Identifiers: Block Header Hash and Block Height
 
-((("blockchain technology", "block identifiers")))((("blocks", "block height")))((("blocks", "block hash")))The primary identifier of a block is its cryptographic hash, a digital fingerprint, made by hashing the block header twice through the SHA256 algorithm. The resulting 32-byte hash is called the _block hash_ but is more accurately the _block header hash_, pass:[<span role="keep-together">because only the block header is used to compute it. For example,</span>] +000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f+ is the block hash of the first bitcoin block ever created. The block hash identifies a block uniquely and unambiguously and can be independently derived by any node by simply hashing the block header.
+((("blockchain (the)", "block identifiers")))((("blocks", "block height")))((("blocks", "block hash")))The primary identifier of a block is its cryptographic hash, a digital fingerprint, made by hashing the block header twice through the SHA256 algorithm. The resulting 32-byte hash is called the _block hash_ but is more accurately the _block header hash_, pass:[<span role="keep-together">because only the block header is used to compute it. For example,</span>] +000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f+ is the block hash of the first bitcoin block ever created. The block hash identifies a block uniquely and unambiguously and can be independently derived by any node by simply hashing the block header.
 
 Note that the block hash is not actually included inside the block's data structure, neither when the block is transmitted on the network, nor when it is stored on a node's persistence storage as part of the blockchain. Instead, the block's hash is computed by each node as the block is received from the network. The block hash might be stored in a separate database table as part of the block's metadata, to facilitate indexing and faster retrieval of blocks from disk.
 
@@ -68,7 +68,7 @@ A block's _block hash_ always identifies a single block uniquely. A block also a
 
 === The Genesis Block
 
-((("blocks", "genesis block")))((("blockchain technology", "genesis block")))The first block in the blockchain is called the genesis block and was created in 2009. It is the common ancestor of all the blocks in the blockchain, meaning that if you start at any block and follow the chain backward in time, you will eventually arrive at the genesis block.
+((("blocks", "genesis block")))((("blockchain (the)", "genesis block")))The first block in the blockchain is called the genesis block and was created in 2009. It is the common ancestor of all the blocks in the blockchain, meaning that if you start at any block and follow the chain backward in time, you will eventually arrive at the genesis block.
 
 Every node always starts with a blockchain of at least one block because the genesis block is statically encoded within the bitcoin client software, such that it cannot be altered. Every node always "knows" the genesis block's hash and structure, the fixed time it was created, and even the single transaction within. Thus, every node has the starting point for the blockchain, a secure "root" from which to build a trusted blockchain.
 
@@ -116,7 +116,7 @@ The genesis block contains a hidden message within it. The coinbase transaction
 
 === Linking Blocks in the Blockchain
 
-((("blocks", "linking blocks in the blockchain")))((("blockchain technology", "linking blocks in the blockchain")))Bitcoin full nodes maintain a local copy of the blockchain, starting at the genesis block. The local copy of the blockchain is constantly updated as new blocks are found and used to extend the chain. As a node receives incoming blocks from the network, it will validate these blocks and then link them to the existing blockchain. To establish a link, a node will examine the incoming block header and look for the "previous block hash."
+((("blocks", "linking blocks in the blockchain")))((("blockchain (the)", "linking blocks in the blockchain")))Bitcoin full nodes maintain a local copy of the blockchain, starting at the genesis block. The local copy of the blockchain is constantly updated as new blocks are found and used to extend the chain. As a node receives incoming blocks from the network, it will validate these blocks and then link them to the existing blockchain. To establish a link, a node will examine the incoming block header and look for the "previous block hash."
 
 Let's assume, for example, that a node has 277,314 blocks in the local copy of the blockchain. The last block the node knows about is block 277,314, with a block header hash of:
 
@@ -154,7 +154,7 @@ Looking at this new block, the node finds the +previousblockhash+ field, which c
 [[merkle_trees]]
 === Merkle Trees
 
-((("merkle trees", id="merkle09")))((("blockchain technology", "merkle trees", id="BCTmerkle09")))Each block in the bitcoin blockchain contains a summary of all the transactions in the block using a _merkle tree_.
+((("merkle trees", id="merkle09")))((("blockchain (the)", "merkle trees", id="BCTmerkle09")))Each block in the bitcoin blockchain contains a summary of all the transactions in the block using a _merkle tree_.
 
 ((("binary hash trees", see="merkle trees")))A _merkle tree_, also known as a _binary hash tree_, is a data structure used for efficiently summarizing and verifying the integrity of large sets of data. Merkle trees are binary trees containing cryptographic hashes. The term "tree" is used in computer science to describe a branching data structure, but these trees are usually displayed upside down with the "root" at the top and the "leaves" at the bottom of a diagram, as you will see in the examples that follow.
 
@@ -268,7 +268,7 @@ Consider, for example, an SPV node that is interested in incoming payments to an
 
 === Bitcoin's Test Blockchains
 
-((("blockchain technology", "test blockchains", id="BCTtest09")))((("mainnet", seealso="blockchain technology")))You might be surprised to learn that there is more than one bitcoin blockchain. The "main" bitcoin blockchain, the one created by Satoshi Nakamoto on January 3rd, 2009, the one with the genesis block we studied in this chapter, is called _mainnet_.  There are other bitcoin blockchains that are used for testing purposes: at this time _testnet_, _segnet_, and _regtest_. Let's look at each in turn.((("testnet", id="testnet09")))
+((("blockchain (the)", "test blockchains", id="BCTtest09")))((("mainnet", seealso="blockchain (the)")))You might be surprised to learn that there is more than one bitcoin blockchain. The "main" bitcoin blockchain, the one created by Satoshi Nakamoto on January 3rd, 2009, the one with the genesis block we studied in this chapter, is called _mainnet_.  There are other bitcoin blockchains that are used for testing purposes: at this time _testnet_, _segnet_, and _regtest_. Let's look at each in turn.((("testnet", id="testnet09")))
 
 
 ==== Testnet&#x2014;Bitcoin's Testing Playground

ch10.asciidoc

@@ -326,7 +326,7 @@ In a coinbase transaction, the first two fields are set to values that do not re
 
 ((("coinbase transactions", "coinbase data")))Coinbase transactions do not have an unlocking script (aka, +scriptSig+) field. Instead, this field is replaced by coinbase data, which must be between 2 and 100 bytes. Except for the first few bytes, the rest of the coinbase data can be used by miners in any way they want; it is arbitrary data.
 
-((("nonce values")))((("blocks", "genesis block")))((("blockchain technology", "genesis block")))((("genesis block")))In the genesis block, for example, Satoshi Nakamoto added the text "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks" in the coinbase data, using it as a proof of the date and to convey a message. Currently, miners use the coinbase data to include extra nonce values and strings identifying the mining pool.
+((("nonce values")))((("blocks", "genesis block")))((("blockchain (the)", "genesis block")))((("genesis block")))In the genesis block, for example, Satoshi Nakamoto added the text "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks" in the coinbase data, using it as a proof of the date and to convey a message. Currently, miners use the coinbase data to include extra nonce values and strings identifying the mining pool.
 
 The first few bytes of the coinbase used to be arbitrary, but that is no longer the case. As per BIP-34, version-2 blocks (blocks with the version field set to 2) must contain the block height index as a script "push" operation in the beginning of the coinbase field.
 
@@ -366,7 +366,7 @@ $ ./satoshi-words
 
 === Constructing the Block Header
 
-((("mining and consensus", "constructing block headers")))((("blocks", "headers")))((("headers")))((("blockchain technology", "block headers")))To construct the block header, the mining node needs to fill in six fields, as listed in <<block_header_structure_ch10>>.
+((("mining and consensus", "constructing block headers")))((("blocks", "headers")))((("headers")))((("blockchain (the)", "block headers")))To construct the block header, the mining node needs to fill in six fields, as listed in <<block_header_structure_ch10>>.
 
 [[block_header_structure_ch10]]
 .The structure of the block header
@@ -394,7 +394,7 @@ At the time that block 277,316 was mined, the version number describing the bloc
 By selecting the specific _parent_ block, indicated by the Previous Block Hash field in the candidate block header, Jing is committing his mining power to extending the chain that ends in that specific block. In essence, this is how Jing "votes" with his mining power for the longest-difficulty valid chain.
 ====
 
-((("merkle trees")))((("blockchain technology", "merkle trees")))The next step is to summarize all the transactions with a merkle tree, in order to add the merkle root to the block header. The coinbase transaction is listed as the first transaction in the block. Then, 418 more transactions are added after it, for a total of 419 transactions in the block. As we saw in the <<merkle_trees>>, there must be an even number of "leaf" nodes in the tree, so the last transaction is duplicated, creating 420 nodes, each containing the hash of one transaction. The transaction hashes are then combined, in pairs, creating each level of the tree, until all the transactions are summarized into one node at the "root" of the tree. The root of the merkle tree summarizes all the transactions into a single 32-byte value, which you can see listed as "merkle root" in <<block277316>>, and here:
+((("merkle trees")))((("blockchain (the)", "merkle trees")))The next step is to summarize all the transactions with a merkle tree, in order to add the merkle root to the block header. The coinbase transaction is listed as the first transaction in the block. Then, 418 more transactions are added after it, for a total of 419 transactions in the block. As we saw in the <<merkle_trees>>, there must be an even number of "leaf" nodes in the tree, so the last transaction is duplicated, creating 420 nodes, each containing the hash of one transaction. The transaction hashes are then combined, in pairs, creating each level of the tree, until all the transactions are summarized into one node at the "root" of the tree. The root of the merkle tree summarizes all the transactions into a single 32-byte value, which you can see listed as "merkle root" in <<block277316>>, and here:
 
 ----
 c91c008c26e50763e9f548bb8b2fc323735f73577effbc55502c51eb4cc7cf2e
@@ -753,7 +753,7 @@ In the next section we will look at how discrepancies between competing chains (
 [[forks]]
 ==== Blockchain Forks
 
-((("mining and consensus", "assembling and selecting chains of blocks", "blockchain forks")))((("blockchain technology", "blockchain forks", id="BCTfork10")))((("forks", "blockchain fork events", id="forks10")))Because the blockchain is a decentralized data structure, different copies of it are not always consistent. Blocks might arrive at different nodes at different times, causing the nodes to have different perspectives of the blockchain. To resolve this, each node always selects and attempts to extend the chain of blocks that represents the most Proof-of-Work, also known as the longest chain or greatest cumulative work chain. By summing the work recorded in each block in a chain, a node can calculate the total amount of work that has been expended to create that chain. As long as all nodes select the greatest-cumulative-work chain, the global bitcoin network eventually converges to a consistent state. Forks occur as temporary inconsistencies between versions of the blockchain, which are resolved by eventual reconvergence as more blocks are added to one of the forks.
+((("mining and consensus", "assembling and selecting chains of blocks", "blockchain forks")))((("blockchain (the)", "blockchain forks", id="BCTfork10")))((("forks", "blockchain fork events", id="forks10")))Because the blockchain is a decentralized data structure, different copies of it are not always consistent. Blocks might arrive at different nodes at different times, causing the nodes to have different perspectives of the blockchain. To resolve this, each node always selects and attempts to extend the chain of blocks that represents the most Proof-of-Work, also known as the longest chain or greatest cumulative work chain. By summing the work recorded in each block in a chain, a node can calculate the total amount of work that has been expended to create that chain. As long as all nodes select the greatest-cumulative-work chain, the global bitcoin network eventually converges to a consistent state. Forks occur as temporary inconsistencies between versions of the blockchain, which are resolved by eventual reconvergence as more blocks are added to one of the forks.
 
 [TIP]
 ====