What is a shave and a haircut knock?

A shave and a haircut knock is a rhythmic pattern of knocks commonly used to signal the end of a performance or to get someone's attention. It consists of two short knocks followed by one long knock and is typically used as a cue for someone to respond with "two bits," an expression meaning "25 cents" and referencing the cost of a traditional barber shop service of a shave and a haircut. The origin of the shave and a haircut knock is unclear, but historians believe it may have emerged in the 19th century as a way for barbers to signal the end of a client's shave and haircut. Today, it is often used as a friendly greeting among musicians, performers, and friends.