Sometimes you come across the indication alt. This means the chord is altered. But what noted should we play sharp or flat? The funny thing is that even musicians writing this chord do not really know what it means. Most jazz musicians agree it means something like this:
The alt chord is a dominant chord
(thus, it is a major chord with a seventh),
probably it has a flat ninth
and probably it has a sharp fifth,
but it might have a sharp ninth instead of a flat ninth,
and it might have a flat fifth instead of a sharp fifth,
and then it may have both, that is,
it might have a flat ninth plus a sharp ninth,
and it might have a sharp fifth plus a flat fifth.
To my knowledge this is the only chord that has so much ambiguity in it. I also found a definition saying that in this chord both the fifth and the ninth are altered. That makes sense.
So let’s recapitulate: the
Galt is something like
G7,<9,<5 or even
G7>9,<9,>5,<5. No wonder someone thought of an abbreviation.