This is Beaumont Rag.

There are several rags that are popular apart from Black Mountain but this one is probably the best.

This tune is usually played in D - capo 2.

There are some absolutely brilliant versions - not only the Clarence White and Doc Watson versions (Tony Rice never did much with it, strangely) but Alison Krauss' band Union Station did a hot one. My favorite is one by a player called Michael Aumen. It's got some wonderful stuff in it which I must work out sometime!

But this is the standard version - A part.