I'm covering the low end at a four-piece jam. Guitar, bass, drums, keys. I had been listening to "Waiting Room" by Fugazi, and I had worked out the opening riff. I liked the sound, I liked the range. We stop a song, I call out the key, and we go in.
The keyboard player takes a step back from the keys.
Why? Why did that simple choice alienate that musician? I can explain this with a little bit of theory. Not much, don't worry.
Consider the Circle of Fifths. Starting at C and going right, you see all the sharp keys (G, D, A, E, B), and going left, you see all the flat keys (F, Bb, Eb, Ab, Db). If you've been playing a little while, you will look at the first four sharp keys and feel comfortable. Guitarists, especially guitarists used to playing first position, open string stuff, are very comfortable with C, G, D, A and E. Things just lay out perfectly on guitar with those keys, because you have a maximum number of open strings. Guitarists tend to gripe when you get into the flats. "F? F!"
The good point is that, once you forget about open strings and play closed positions, things get easier. If you barre every chord, a Bb is like an A played up one fret. Everything is pretty much parallel. The core concept of CAGED theory is that once you can play a scale in one of five positions, you can move that scale to another position and get another scale. You have to learn one thing five ways and you pretty much have it in every key. On bass, I was thinking of F# as G minus one fret.
Now, let's look at it like a keyboardist. That scale is F#, G#, A#, B, C#, D# and E#, which is F. Writing that on staff, you'd mark it with six sharps, or six flats if you're thinking of it as Gb instead. There are only five black keys on the keyboard. And because the keyboard layout is not uniform, the scales and chords are not as instantly transferrable between keys.
By calling out F#, I pushed the keyboards to the most foreign landscape possible.
Every instrument has tendencies like that. For example, horns love the flats, and if you ever want to learn a sax lick or figure out a song with a big horn chart, always look for Bb first.
A man's got to know his limitations, and it's good to know the limitations of your band members, too.