diff --git a/doc/lispref/files.texi b/doc/lispref/files.texi
index 70db8521481..5c63f192447 100644
--- a/doc/lispref/files.texi
+++ b/doc/lispref/files.texi
@@ -1299,7 +1299,8 @@ and modification.
 This function returns @code{t} if the file @var{filename1} is
 newer than file @var{filename2}.  If @var{filename1} does not
 exist, it returns @code{nil}.  If @var{filename1} does exist, but
-@var{filename2} does not, it returns @code{t}.
+@var{filename2} does not, it returns @code{t}.  Otherwise, it compares
+the times of last modification of the files.
 
 In the following example, assume that the file @file{aug-19} was written
 on the 19th, @file{aug-20} was written on the 20th, and the file
diff --git a/src/fileio.c b/src/fileio.c
index 7afe3e75737..2db2760916b 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -3788,7 +3788,8 @@ DEFUN ("unix-sync", Funix_sync, Sunix_sync, 0, 0, "",
 DEFUN ("file-newer-than-file-p", Ffile_newer_than_file_p, Sfile_newer_than_file_p, 2, 2, 0,
        doc: /* Return t if file FILE1 is newer than file FILE2.
 If FILE1 does not exist, the answer is nil;
-otherwise, if FILE2 does not exist, the answer is t.  */)
+otherwise, if FILE2 does not exist, the answer is t.
+For existing files, this compares their last-modified times.  */)
   (Lisp_Object file1, Lisp_Object file2)
 {
   struct stat st1, st2;