Why is jazz sheet music commonly available in C, E-flat, and B-flat?
Sheet music written out in those three keys (and bass clef in "C") covers all variations of instruments common to jazz: "C" covers keyboards, bass (usually the bass would just improvise his/her part from the chords on the lead sheet), guitar, vibes, voice, and flute. "Bb" covers the trumpet, soprano and tenor saxes, and flugelhorn. "Eb" would be for the alto and baritone saxes. Usually, BTW, there is bass clef sheet music as well; this would be for the trombonist mainly, though the bassist could use this as well (though it is not strictly necessary for him/her in most cases, unless he/she is to read the melody at some point). The chords almost always included above the melody (or at the bottom of the page) to aid in improvisation on all the parts. Drummers are usually given a "C" for a form/meter change reference, if they need any music at all.