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Merge pull request #791 from rating89us/patch-30

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ch02: explain that both outputs are in a single tx
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Will Binns 2021-02-20 10:04:24 +00:00 committed by GitHub
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@ -114,7 +114,7 @@ image::images/mbc2_0204.png["Transaction chain"]
==== Making Change
((("change, making")))((("change addresses")))((("addresses", "change addresses")))Many bitcoin transactions will include outputs that reference both an address of the new owner and an address of the current owner, called the _change_ address. This is because transaction inputs, like currency notes, cannot be divided. If you purchase a $5 US dollar item in a store but use a $20 US dollar bill to pay for the item, you expect to receive $15 US dollars in change. The same concept applies to bitcoin transaction inputs. If you purchased an item that costs 5 bitcoin but only had a 20 bitcoin input to use, you would send one output of 5 bitcoin to the store owner and one output of 15 bitcoin back to yourself as change (less any applicable transaction fee). Importantly, the change address does not have to be the same address as that of the input and for privacy reasons is often a new address from the owner's wallet.
((("change, making")))((("change addresses")))((("addresses", "change addresses")))Many bitcoin transactions will include outputs that reference both an address of the new owner and an address of the current owner, called the _change_ address. This is because transaction inputs, like currency notes, cannot be divided. If you purchase a $5 US dollar item in a store but use a $20 US dollar bill to pay for the item, you expect to receive $15 US dollars in change. The same concept applies to bitcoin transaction inputs. If you purchased an item that costs 5 bitcoin but only had a 20 bitcoin input to use, your wallet would create a single transaction that sends two outputs, one output of 5 bitcoin to the store owner and one output of 15 bitcoin back to yourself as change (less any applicable transaction fee). Importantly, the change address does not have to be the same address as that of the input and for privacy reasons is often a new address from the owner's wallet.
Different wallets may use different strategies when aggregating inputs to make a payment requested by the user. They might aggregate many small inputs, or use one that is equal to or larger than the desired payment. Unless the wallet can aggregate inputs in such a way to exactly match the desired payment plus transaction fees, the wallet will need to generate some change. This is very similar to how people handle cash. If you always use the largest bill in your pocket, you will end up with a pocket full of loose change. If you only use the loose change, you'll always have only big bills. People subconsciously find a balance between these two extremes, and bitcoin wallet developers strive to program this balance.