diff --git a/Doc/lib/libstringio.tex b/Doc/lib/libstringio.tex
index c268690..f78d016 100644
--- a/Doc/lib/libstringio.tex
+++ b/Doc/lib/libstringio.tex
@@ -62,7 +62,8 @@
 Another difference from the \refmodule{StringIO} module is that calling
 \function{StringIO()} with a string parameter creates a read-only object.
 Unlike an object created without a string parameter, it does not have
-write methods.
+write methods.  These objects are not generally visible.  They turn up in
+tracebacks as \class{StringI} and \class{StringO}.
 
 The following data objects are provided as well: