Conversation so far:
What did Charlie Parker do in Charlie Parker's early career?
In the late 1930s Parker began to practice diligently.
During this period he mastered.
What did he master?
improvisation and developed some of the ideas that led to bebop.
Did he work with anyone else of note?
1938 Parker joined pianist Jay McShann's territory band.

Question: What did they do together?
Self-contained rewrite: What did Parker and Jay McShann do together?