Depends on what else they play.

If a person who can play vibes, drums, and other assorted percussion instruments decides to play the tambourine and/or shaker I call him/her a percussionist.

If the sax player picks up the tambourine and plays it when there is no sax part, I still call him/her a sax player.

If the sax player also plays many percussion instruments like marimba, trap kit, etc., I'll call him/her a multi-instrumentalist.

If the producer decides to put cowbell in there, I'll still call him/her a producer and might add one who makes some creative suggestions.

I started out as a drummer but quickly switched and became a sax player. But being in a rock band required me to wear many hats. So I'd pick up the tambourine, shaker, etc. Probably the first 'other' instrument was bass. The bass player in an early band was also a guitarist, so on those non-sax songs he'd show me the bass line and he'd play second guitar. Then came keys, guitar and toys (shaker, cowbell, etc.)

It wasn't until I got good at the bass, guitar, and keyboard that I considered myself a multi-instrumentalist. But while playing the toys, I never considered myself a percussionist, even though I can actually play drums.

In music, it's so hard to label a lot of things and people.

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