Glossary
What is a sit in?
To sit in is to join another act on stage for a song or two without being a member of that band — common at jams, residencies, and when a temporary band member is needed to cover.
Sitting in is usually arranged in advance with the bandleader, and it is normal to be told the key and the arrangement moments before you start.
This is where transposable chord charts earn their keep: a guest horn player or singer needs the tune in a key that works for them, not the key the band happened to write it in.