The mainplate is the flat main metal plate that forms the base of a watch movement. The other parts of the movement are mounted on it, and it holds them together. With the bridges fixed to it, it makes up the frame of the watch.
Where does the mainplate sit in a watch?
The mainplate sits directly behind the dial. It is always round, or it matches the shape and size of the movement. In pocket watches it is called the pillar plate or the bottom plate, and every component attaches to it. Because it sits under the dial, it comes into view once the dial is taken off.
What does the mainplate do?
The mainplate keeps the parts of the movement together and forms its base. One watchmaking writer likens it to the chassis of a car: the base on which the watchmaker builds everything else. In a traditional movement, the wheels and other moving parts are held between two plates, a front plate and a back plate. In modern movements, the back plate is replaced by separate bars called bridges, but the front plate is not. The bridges are fixed to the mainplate, and together they form the frame of the watch.
Where it comes from
The calibre of Jean-Antoine Lépine changed how the mainplate was used. Older movements relied on pillars to hold upper plates above the main plate. Lépine did away with the top plate. He kept a single plate and mounted separate cocks on it. Bars and bridges took the place of the pillars and upper plates.
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