Patek Philippe Ref. 5101P “10 Day Tourbillon” with double barrel and 10 days power reserve
© Patek Philippe
As watches became more complicated with the advent of the chronograph, repeater, and other complications, the need arose for larger power reserve. Since a mainspring can only hold so much energy, watchmakers began adding a second barrel and mainspring. Today, some watch movements even include three or four barrels to power their complications or give longer running time.
Multiple barrels can be configured in three different ways