Fourth wheel is one of the three moving parts in the gear train of a mechanical watch. It is located between the third wheel and the escapement wheel which is part of the escapement.

In a traditional wheel, the fourth wheel makes one revolution in 60 seconds. So it is sometimes used to put a second hand directly on its axis. When this is the case, it can often be identified by a long pivot that will be used to place the seconds hand.

In a traditional confirguration, which was the first one used, the second will appear on the dial not in the center but on a separate dial usually placed at six o’clock. This wheel can therefore often be quite easily located in the movement by its position at six o’clock.

Also, it is important to know that this wheel will turn counterclockwise when seen from the movement side. So it will turn clockwise on the dial side.

Although often called the fourth wheel, it is actually a mobile composed of two parts, a wheel generally made of brass and a pinion and its pivots made of steel.