I just want to add a quick clarification note (ha, ha) to this tread about transposing instruments for some future reader who is confused by transposing instruments
Instruments like saxophone, clarinet, trumpet, and many more are considered to be "transposing" because the notes they play don't sound the same pitch as the ones played by a piano or guitar. For the purpose of this short discussion, we will consider a piano and a tenor saxophone playing a "C" note from a sheet of music. The piano part is easy: see a "C", play a "C" and it sounds like a "C". However, for the tenor, if it sees a "C" on a piece of sheet music and plays it the resulting sound will be "Bb". (As an aside, the reason a tenor sax is often called a Bb sax is that when it plays a "C" it sounds a "Bb".)
To get around this problem, sheet music for transposing instruments is transposed so that what you see sounds right. For example, that "C" will appear on the tenor sax chart as a "D". The player fingers his "D" note and out comes a beautiful "C". An alto sax player would have a different sheet of music in which the "C" appears as an "A". A French Horn player has yet another piece of music on which that same note is notated as a "G". In this manner all the "hard work" is done by the sheet music engraver.
In most cases MMA really doesn't care about any of this. It generates MIDI files based on the chords and notes you specify in the source file. If you say play a "C", it plays a "C". No problems. And if you are a sax player your sheet music should be transposed into the correct notation for your instrument.
To further complicate life, I posted an essay a number of years ago at
https://mellowood.ca/music/essays/mma/mma-practice-tool.html suggesting how to use MMA to generate a play-a-long file. In it I transposed the music up 2 semi-tones, assuming that the student only had one copy of the music, and not a concert-pitch sheet and a Bb-sheet. Hey, I'm entitled to make mistakes
The transposing should be DOWN 2 semi-tones. This will soon be corrected in the example file.
Just one more thing: Nothing in the above discussion has anything to do with Key Signatures.