diff --git a/ch11_blockchain.adoc b/ch11_blockchain.adoc
index 7d792e06..6487ebe9 100644
--- a/ch11_blockchain.adoc
+++ b/ch11_blockchain.adoc
@@ -170,7 +170,7 @@ process and will be discussed in more detail in <<mining>>.
 [[block_hash]]
 === Block Identifiers: Block Header Hash and Block Height
 
-The primary identifier of a block
+The ((("blocks", "identifiers", id="block-identify")))((("block header hash", id="block-header-hash")))((("block height", id="block-height")))primary identifier of a block
 is its cryptographic hash, a commitment 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
@@ -223,7 +223,7 @@ table for faster retrieval.
 A block's _block hash_ always identifies a single block uniquely. A
 block also always has a specific _block height_. However, it is not
 always the case that a specific block height identifies a single
-block. Rather, two or more blocks might compete for a single position in
+block. Rather, two or more blocks might compete for a single position((("blocks", "identifiers", startref="block-identify")))((("block header hash", startref="block-header-hash")))((("block height", startref="block-height"))) in
 the blockchain.
 ====