diff --git a/Doc/library/email.header.rst b/Doc/library/email.header.rst
index 000d48e..e424dff 100644
--- a/Doc/library/email.header.rst
+++ b/Doc/library/email.header.rst
@@ -74,7 +74,8 @@
 
    Optional *continuation_ws* must be :rfc:`2822`\ -compliant folding whitespace,
    and is usually either a space or a hard tab character. This character will be
-   prepended to continuation lines.
+   prepended to continuation lines.  *continuation_ws* defaults to a single
+   space character (" ").
 
    Optional *errors* is passed straight through to the :meth:`append` method.