The barrel is the energy reservoir of a mechanical movement: a metal cylinder that contains the coiled mainspring, holds it in tension during winding, and releases it in a controlled manner toward the gear train. Without the barrel, there is no mechanical movement. It is the starting point of everything.
A single barrel is the historical solution and remains the most common today. When manufacturers want to increase the power reserve without enlarging the movement's diameter, the almost inevitable answer is to add a second barrel — sometimes a third. But the way these cylinders are connected to each other fundamentally changes the character of the movement.
In parallel, the two barrels work simultaneously with longer, thinner springs. The direct consequence is more linear torque over time: the energy delivered decreases less abruptly during discharge, and the movement maintains more regular timekeeping. This is the configuration chosen, among others, by Chopard in the L.U.C 96.40-L caliber and by Oris in the Calibre 400, both with power reserves of 65 and 120 hours respectively.
In series, the barrels discharge successively rather than simultaneously. The result is a very long power reserve — Blancpain's caliber 1315 with three barrels in series reaches 120 hours — but it requires more careful tension management to avoid irregularities in torque during the transition phases between barrels.
There is also the vertically stacked configuration, where two barrels share the same axis on overlapping planes. This is the preferred solution in ultra-thin movements because it doesn't consume surface space on the plate: the caliber 2550, for example, achieves 80 hours of autonomy while keeping the total thickness within just a few tenths of a millimeter additional.
Some manufacturers, like De Bethune, have developed self-regulating barrel systems, where tension automatically balances between the two cylinders during discharge. The practical advantage is a more stable power reserve at the extremes of the curve — those zones at the beginning and end — where torque tends to be less predictable.
From the workbench, I've learned one thing: when a salesman mentions multiple barrels as a selling point, I always ask which configuration they're mounted in. Parallel, series, and stacked are not the same thing, and the answer tells you a lot about what the caliber's designer really had in mind.