diff --git a/ch05_wallets.adoc b/ch05_wallets.adoc
index e9e5235f..5d7d1632 100644
--- a/ch05_wallets.adoc
+++ b/ch05_wallets.adoc
@@ -1207,7 +1207,7 @@ doesn't "run out" of keys.
 
 .Mind the Gap
 ****
-An extended public key can generate approximately 4 billion direct
+An ((("gap limit", id="gap-limit")))extended public key can generate approximately 4 billion direct
 child keys, far more than any store or application should ever need.
 However, it would also take a wallet application an unreasonable amount
 of time to generate all 4 billion keys and scan the blockchain for
@@ -1250,7 +1250,7 @@ been proposed, such as
 asking the spender's wallet to construct (but not broadcast) a
 transaction paying a possibly reused address before they receive a fresh
 address for the actual transaction.  However, these other solutions have
-not been used in production as of this writing.
+not been used in production as of this((("gap limit", startref="gap-limit"))) writing.
 ****
 
 Another common application of this solution is for