Conversation so far:
When did Gerry Mulligan start working with Chet Baker?
Mulligan began performing on off-nights at The Haig in 1952. a young trumpeter named Chet Baker began sitting in with Mulligan. Mulligan and Baker began recording together
When did Gerry Mulligan and Chet Baker form the quartet?

Question: What instruments were in the quartet?
Self-contained rewrite: What instruments were in Gerry Mulligan's quartet?