diff --git a/README.md b/README.md
index be61141..814eb20 100644
--- a/README.md
+++ b/README.md
@@ -248,9 +248,11 @@ TinyExpr parses the following grammar:
 
 In addition, whitespace between tokens is ignored.
 
-Valid variable names are any combination of the lower case letters *a* through
-*z*. Constants can be integers, decimal numbers, or in scientific notation
-(e.g. *1e3* for *1000*). A leading zero is not required (e.g. *.5* for *0.5*)
+Valid variable names consist of a lower case letter followed by any combination
+of: lower case letters *a* through *z*, the digits *0* through *9*, and
+underscore. Constants can be integers, decimal numbers, or in scientific
+notation (e.g.  *1e3* for *1000*). A leading zero is not required (e.g. *.5*
+for *0.5*)
 
 
 ##Functions supported