#1
Do they provide just the guitar parts to the song? Guitarists like slash, Jimmy Page, Eddie Van Halen, who don't sing, do they ever write vocal parts but let their singer sing the final version?

Also if you don't write the vocals to the song but come up with the chord progression,riffs, and solo, would you get songwriting credit?
#2
i know alot of bands whose instrumentalists write entire songs. cannibal corpse is a good example,i think. If i'm not wrong their vocalist doesn't write anything.
#3
if your in a band, then no, your band alltogether would get credit, unless noted in a gig/cd or PSA (public service announcement), if your solo, or working with another artist/musician(its happened), then you would both take equal credit
#4
I think Jimmy Page helped

(guess)

It depends, really. Some do, some don't
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#5
I've written all the music for every song my two bands have so far. The vocalist just writes lyrics
#6
In my old band I wrote the lyrics for the singer but didn't sing in the band properly.
Effectively I had to write the rhytym part that the singer should be playing to be able to work out how to sing the lyrics that I'd written.
#7
john petrucci writes most lyrics and music for dream theater, same with steve harris from maiden. just depends on how the band works really.

if you've written any part of a song, you get credit. this applies to original material only, following root notes on a bass doesn't count.