diff --git a/doc/bird.sgml b/doc/bird.sgml
index c46ea4f0..2f65f137 100644
--- a/doc/bird.sgml
+++ b/doc/bird.sgml
@@ -712,10 +712,10 @@ incompatible with each other (that is to prevent you from shooting in the foot).
 
 	  Sets of prefixes are special: their literals does not allow ranges, but allows
 	  prefix patterns that are written as <cf><M>ipaddress</M>/<M>pxlen</M>{<M>low</M>,<M>high</M>}</cf>.
-	  Prefix <cf><m>ip1</m>/<m>len1</m></cf> matches prefix pattern <cf><m>ip2</m>/<m>len2</m>{<m>l</m>,<m>h</m>}</cf> iff 
+	  Prefix <cf><m>ip1</m>/<m>len1</m></cf> matches prefix pattern <cf><m>ip2</m>/<m>len2</m>{<m>l</m>,<m>h</m>}</cf> if 
 	  the first <cf>min(len1, len2)</cf> bits of <cf/ip1/ and <cf/ip2/ are identical and <cf>len1 &lt;= ip1 &lt;= len2</cf>.
 	  A valid prefix pattern has to satisfy <cf>low &lt;= high</cf>, but <cf/pxlen/ is not constrained by <cf/low/
-	  or <cf/high/. Obviously, a prefix matches a prefix set literal iff it matches any prefix pattern in the
+	  or <cf/high/. Obviously, a prefix matches a prefix set literal if it matches any prefix pattern in the
 	  prefix set literal.
 
 	  There are also two shorthands for prefix patterns: <cf><m>address</m>/<m/len/+</cf> is a shorthand for