... If you have perfect pitch, you can eventually learn to barbershop. It may take time. The only person I've ever known who could NOT do so had the exact opposite affliction - he could hear things just fine but could NOT sing on the same pitch to save his life. No perfect pitch there.

I ding this poll for no checkboxes, as several of the answers can apply - if I'm making someone else's song bit into a tag, it's with their words and one line out of four from their notes, before I start. If a song lyric hits me, the music can come second, or if I'm musicing an already-given poem (which I haven't done much since childhood). Filking, as noted, usually starts with the music given.

--Dave