diff --git a/doc_src/math.txt b/doc_src/math.txt
index a29a3a834..b58527f97 100644
--- a/doc_src/math.txt
+++ b/doc_src/math.txt
@@ -8,8 +8,13 @@
 
 math is used to perform mathematical calcualtions. It is only a very
 thin wrapper for the bc program, that makes it possible to specify an
-expression from the commandline without using non-standard extensions
+expression from the command line without using non-standard extensions
 or a pipeline. Simply use a command like <code>math 1+1</code>. 
 
 For a description of the syntax supported by math, see the manual for
-the bc program.
+the bc program. Keep in mind that parameter expansion takes place on
+any expressions before they are evaluated. This can be very useful in
+order to perform calculations involving environment variables or the
+output of command substitutions, but it also means that parenthesis
+have to be escaped.
+