The arch-mouth Pelham looks somewhat like a mullen mouth, in that it has a solid bar for a mouthpiece. However, in this case, the whole mouthpiece is bowed slightly upwards, which gives the tongue more room, hence putting more pressure on the bars. This could give more lateral control, while still distributing pressure fairly evenly over the tongue. Because many mullen mouths are made of flexible rubber, one could argue that they have the same effect as a solid arch-mouth, since when the reins are employed, the rubber flexes slightly, leading to the same shape as an arch-mouth.