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

Question: When did they form the quartet?
Self-contained rewrite: When did Gerry Mulligan and Chet Baker form the quartet?