diff --git a/ch04_keys.adoc b/ch04_keys.adoc
index 5b5e0888..089f9015 100644
--- a/ch04_keys.adoc
+++ b/ch04_keys.adoc
@@ -611,11 +611,11 @@ look at compact encoding and reliable ((("public key cryptography", "hash functi
 In order((("public key cryptography", "base58check encoding", id="pub-key-base58")))((("base58check encoding", id="base58")))((("encoding", "base58check", id="encode-base58"))) to represent long numbers in a compact way,
 using fewer symbols, many computer systems use mixed-alphanumeric
 representations with a base (or radix) higher than 10. For example,
-whereas the traditional decimal system uses 10 numerals, 0 through 9,
-the hexadecimal system uses 16, with the letters A through F as the six
+whereas the traditional ((("decimal system")))decimal system uses 10 numerals, 0 through 9,
+the ((("hexadecimal system")))hexadecimal system uses 16, with the letters A through F as the six
 additional symbols. A number represented in hexadecimal format is
 shorter than the equivalent decimal representation. Even more compact,
-base64 representation uses 26 lowercase letters, 26 capital letters, 10
+base64 representation ((("base64 encoding")))uses 26 lowercase letters, 26 capital letters, 10
 numerals, and 2 more characters such as "+" and "/" to
 transmit binary data over text-based media such as email.