Bach’s Trio Sonata for flute, violin and basso continuo from the Musical Offering is probably the pre-eminent trio sonata in music history. When Bach met King Frederick the Great in Potsdam in 1747, the sovereign presented the composer with the celebrated “Royal Theme”, composed by himself. It underpins the entire Musical Offering, which also contains several fugues and canons as well as the Trio Sonata. The Canon perpetuus that follows the Trio can also be played by a trio. The term “perpetuus” here means that the performers themselves are to determine when the canon ends.