Standard gear train in a watch movement; L-R, mainspring barrel, center wheel, third wheel, fourth wheel, escape wheel, lever (balance not shown)
Starting from the mainspring barrel (in the diagram, on the left, and moving from left to right) we have the center wheel, third wheel, fourth wheel, escape wheel (with its distinctive asymmetrical teeth) the lever, and finally the balance and its spring. Two things to bear in mind: the center wheel drives the hands of a watch and is centered right below them, and the fourth wheel, in a typical movement, turns once per minute. That means that if you want a seconds hand, all you have to do is stick a hand on the fourth wheel pivot, and you’re good to go. The fact that in a classic gear train, the fourth wheel is off to one side, is the reason that in, say, a vintage pocket watch, you’ll see the seconds in a subdial (and it’s also the reason that for much of watchmaking, center seconds hands were considered a complication, but that’s another article).