The quarter bells play a once-repeating, 20-note sequence of rounds and four changes in the key of E major: 1–4 at quarter past, 5–12 at half past, 13–20 and 1–4 at quarter to, and 5–20 on the hour (which sounds 25 seconds before the main bell tolls the hour). Because the low bell (B) is struck twice in quick succession, there is not enough time to pull a hammer back, and it is supplied with two wrench hammers on opposite sides of the bell. The tune is that of the Cambridge Chimes, first used for the chimes of Great St Mary's church, Cambridge, and supposedly a variation, attributed to William Crotch, based on violin phrases from the air "I know that my Redeemer liveth" in Handel's Messiah. The notional words of the chime, again derived from Great St Mary's and in turn an allusion to Psalm 37:23–24, are: "All through this hour/Lord be my guide/And by Thy power/No foot shall slide".Lockyer, p. 149. They are written on a plaque on the wall of the clock room.
