The Westminster chime is not a simple hourly chime: it is a melody articulated in four distinct phrases, each corresponding to a quarter hour, built on four notes that concatenate in different order at each quarter. Four hammers, four gongs. At the quarter, four notes sound; at the half, eight; at three quarters, twelve; at the full hour, sixteen notes followed by the hour strikes. Anyone familiar with the Westminster bells in London recognizes the progression immediately.
From a mechanical standpoint, the difference from a traditional repeater is substantial. The classical repeater works with two strikers — hours and minutes, or hours, quarters and minutes — and the logic of the racks is relatively linear. The Westminster carillon requires four hammers synchronized in precise sequence, with a rhythmic grammar that the movement must encode in cams and command blocks. Every error in the note sequence is audible. The margin of tolerance is zero.
The complication is almost always combined with Grande Sonnerie or Petite Sonnerie. In Grande Sonnerie, the watch automatically strikes the quarters with the hours repeated at each quarter; in Petite Sonnerie it strikes the quarters without repeating the hours. The manually activated Westminster minute repeater then adds the remaining minutes after the last quarter. In grand complications — the Vacheron Constantin Les Cabinotiers Solaria counts 41 complications, of which seven patents relate solely to the Westminster repeater mechanism — the design challenge concerns the coexistence of this sound mechanism with chronograph, tourbillon and regulator within the confines of a single movement.
The problem of sound quality is often underestimated. The gongs must vibrate freely to resonate: the way they are anchored to the case or caseback determines the tonal quality as much as their shape. In the Solaria the hammers have been redesigned in steel and gold to increase mass and improve the transfer of kinetic energy. In other calibers the gongs are fixed to the bezel or intermediate caseback to amplify propagation. There is no universal solution: each case requires its own acoustic solution.
It is worth noting that not all Westminster carillons play the same melody. The traditional sequence is codified, but some makers deliberately depart from it. The Bulgari Octo Roma Grande Sonnerie replaces the Westminster quarters with a tritone interval — three whole tones, a classical dissonance — developed in collaboration with an orchestra conductor. It is a precise aesthetic choice, not a defect. Knowing the original sequence helps to understand when you are facing an intentional variant.