"Alt" chord is a common designator, typically used in jazz but can be used for a few other genres at times, and it means that the player has a choice as to how to alter the chord.

The alteration is always either b5, #5, b9, #9, with the latter two maybe used less often, it is also not uncommon to play two of the alterations at the same time in the same ALT chord, say, b5 and #9, together, as well. Or three of them (!) as in G7b5,#5,b9. Spelling, G,B,Db,D#,F,G#. Avoid close harmonies, though, best used in open voicings most of the time.

Typically used with dom7 in there also.

There are rules governing which one you might pick at a given time, with the #5/b5 bing "kind of" interchangeable, depending upon a few other things such as Melody note or plain old musical taste.

Of very good use as transitions, as opposed to those chords one might "land on".

http://en.wikipedia.org/wiki/Altered_chord


--Mac

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