If you play an instrument that's not in concert key you know that the music you're reading isn't showing the actual pitch coming out of
Typically you just have music for your instrument and you don't think about it. But if you want to jam with friends or play with
the piano (or guitar or violin) using their music you have to transpose.
I've wasted a lot of time making various transposing wheels
to visualize the relationship between the music you're looking at and the key you're actually playing in.
The problem with those is that at the moment you need them, they are bothersome to use.
They are useful for education purposes (although an app on your phone might work better).
To quickly determine what key an instrument is in, I came up with a kind of
goofy mnemonic that works much better for me.
Arrange the Instruments in Fifths
The key principle is to arrange the instruments in fifths, like the circle of fifths. Piano (C), Horn in F, Bb Trumpet, Eb alto sax.
Wait, but C-F-Bb-Eb . . . that sequence goes up in fourths. Yes, but it goes down in fifths, so that's the direction the music they play from goes up.
This trick is for remembering that order.
This method requires an understanding of the circle of fifths--which is a good tool to have
under your belt anyway if you're serious about music. If you need a refresher in the concept
you can find much better explanations on the internet than I can give you.
The Mnemonic--move from flat to sharp
The basic idea is to start with the concert key, which is represented by the piano, which has a broad "flat" surface.
The next instrument in the series is the French horn, which obviously isn’t as flat as a piano. So the written music for the French horn is one
step sharper than the piano (concert key). To find the notated key for the French Horn you move one step more sharp around the circle of fifths.
The trumpet represents the B-flat instruments. Its shape is more pointy—sharper—than the French Horn.*
Music for Bb instruments are the next step more sharp in the series.**
The sharpest step in our series is the Eb alto sax. Think of the pointy sax mouthpiece. The alto sax music is
three steps sharper around the circle of fifths from the piano.
To find the piano (concert) key you just work backwards. If you're playing in G on the alto sax the piano is
three steps more flat (G-sax, C-trumpet), F-French horn, Bb-concert).
I hope you find this helpful. There's another explanation of this memory aid here.
Honestly this is a lot of words to explain it. I think you'll find this is one of those things that is easier done than said.