It's The Green Mill. The legendary Green Mill, where Al Capone himself frequented. You can go up to the servers or bouncer and have them point out his booth or bullet holes that are still evident to this day. The artists who perform here are gifted beyond miracles. You listen to them and it's like you are in a different world.
It is the most popular place for music in Chicago. Chicago artists themselves come to this joint to appreciate true talent. It is known all over the world however it still manages to give you the feel of an intimate bar and well kept secret by locals.
Bring cash for the cover. Try to refrain from talking during the performance you will get asked to leave otherwise. Bring someone special to impress