diff --git a/asterix-doc/src/site/markdown/aql/manual.md b/asterix-doc/src/site/markdown/aql/manual.md
index da51dee..dbb2b25 100644
--- a/asterix-doc/src/site/markdown/aql/manual.md
+++ b/asterix-doc/src/site/markdown/aql/manual.md
@@ -607,8 +607,17 @@
 
     create index fbSenderLocIndex on FacebookMessages(sender-location) type rtree;
 
+The following example creates a 3-gram index called fbUserIdx on the name field of the FacebookUsers dataset.
+This index can be used to accelerate some similarity or substring maching queries on the name field.
+For details refer to the [document on similarity queries](similarity.html#NGram_Index).
+
+##### Example
+
+    create index fbUserIdx on FacebookUsers(name) type ngram(3);
+
 The following example creates a keyword index called fbMessageIdx on the message field of the FacebookMessages dataset.
-This index can be useful for accelerating text searches involving the message field.
+This keyword index can be used to optimize queries with token-based similarity predicate on the message field.
+For details refer to the [document on similarity queries](similarity.html#Keyword_Index).
 
 ##### Example