diff --git a/doc/formats.txt b/doc/formats.txt
index 578b950..044b325 100644
--- a/doc/formats.txt
+++ b/doc/formats.txt
@@ -135,6 +135,8 @@ POSTGAP
 	``postgap``
 		time in MSF format.
 
+Postgap is filled with generated silence.
+
 PREGAP
 ~~~~~~