+.SH "COMPATIBILITY NOTES"
+Once upon a time, there was only the
+.BR md5sum (1)
+utility. As its name suggested, it calculated MD5 hashes of files. MD5
+was shown to be weak, so the author wrote
+.B hashsum
+to do the same job with other, hopefully stronger, hash functions. The
+original
+.B hashsum
+program tried hard to be compatible with GNU
+.BR md5sum (1),
+but the latter has itself changed in incompatible ways since then;
+.B hashsum
+has intentionally not changed to match.
+.PP
+The following
+.B hashsum
+features are not found in the GNU Coreutils hashing utilities.
+.hP
+Filename escaping (the
+.B \-e
+option).
+.hP
+Magic comment lines in hash data to indicate algorithm selection, hash
+encoding, and filename escaping.
+.hP
+Base-64 and Base-32 output.
+.PP
+Other differences are as follows.
+.hP
+Originally, if GNU
+.B md5sum
+was invoked without any filename arguments, it would print only the hash
+of its stdin to stdout, which was very convenient for scripts which
+manipulate hashes in nontrivial ways. This behaviour was later changed,
+and now the GNU Coreutils hashing utilities always print a filename or
+.RB ` \- '
+after the hash. The
+.B hashsum
+program follows the original
+.B md5sum
+behaviour, and doesn't print a filename if no files were listed on the
+command line.