diff --git a/src/dd.c b/src/dd.c
index 7d8c78b85..22fc69ca6 100644
--- a/src/dd.c
+++ b/src/dd.c
@@ -321,7 +321,8 @@ Each KEYWORD may be:\n\
   ucase     change lower case to upper case\n\
   swab      swap every pair of input bytes\n\
   noerror   continue after read errors\n\
-  sync      pad every input block with NULs to ibs-size\n\
+  sync      pad every input block with NULs to ibs-size; when used\n\
+              block or unblock, pad with spaces rather than NULs\n\
 "));
       puts (_("\nReport bugs to <bug-fileutils@gnu.org>."));
     }