The single jointed snaffle is probably one of the most commonly used snaffle bits. The jointed action allows the rider to put pressure on one side of the mouth more than the other, and hence have better control over the lateral flexion of the horse.
A correctly fitting bit should have a fingers width on either side of
the horses mouth. To ensure your new bit fits correctly measure the
length of the mouthpiece from inside of the rings, as illustrated below.