Composition Question

[Zambo]

When a composer pens a song, does he or she write for all instruments, or do they just write the melody and then an arranger or producer take over.

For example when Lennon and McCartney wrote did that include bass, lead guitar work as well in their composing?

[warmleatherette]

When a composer pens a song, does he or she write for all instruments, or do they just write the melody and then an arranger or producer take over.

For example when Lennon and McCartney wrote did that include bass, lead guitar work as well in their composing?

Depends who it is, some write everything (Prince for instance) some just the melody and let the rest of the band come up with the rest.

[Bad Blue 2000]

Usually composed on guitar or piano.
The rhythm section add nothting, apart from keeping time.