git-describe: what is the '-g' prefix good for?

Hello there,

I'm a bit surprised about the output of git-describe: if I'm on a tag,
git-describe tells the tag's name. Fine. If I commit sth. on top of
that, it tells how far I'm away from that tag and the current commit's
SHA1. Very fine. But then the SHA1 is preceeded with a '-g'. What's that
for?

I really don't want to ask for dropping it (I guess all people rely
on it), but does anybody know about the initial intention of that '-g'?
Cannot be easier parsing in porcelain scripts because either way the
actual state is unambiguously.

Just a curious
     Dirk