-files found in it. With two directories, it prints either the files
-common to both DIR1 and DIR2, or with the `not:' prefix, the ones
-present in DIR1 and not in DIR2.
+files found in it.
+
+With two directories, it prints either the files common to both DIR1
+and DIR2 or, with the `not:' prefix, the ones present in DIR1 and not
+in DIR2. The and: prefix is assumed by default and necessary only if
+you have a directory name starting with `not:'.
+
+This command compares files by first comparing their sizes, hence goes
+reasonably fast.
+
+When looking for identical files, \fBfinddup\fP associates a group ID
+to every content, and prints it along the file names. Use the \fB-g\fP
+to switch it off.
+
+Note that
+.B finddup DIR
+is virtually the same as
+.B finddup -i DIR DIR