diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
index fdacf5aff8c455..7af7a7c0d7c47b 100644
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -8,7 +8,7 @@
 
 This module provides access to some variables used or maintained by the
 interpreter and to functions that interact strongly with the interpreter. It is
-always available.
+always available. Unless explicitly noted otherwise, all variables are read-only.
 
 
 .. data:: abiflags