The "mouthpiece" of a horse's bit is the first factor most people think of when assessing the severity and action of the bit. Therefore, it is carefully considered when choosing a bit for a horse. Many mouthpieces are not allowed in certain competitions. Bit mouthpieces may be single jointed, double-jointed, "mullen" (a straight bar), or have an arched port in the center of varying height, with or without joints. Some have rollers, rings or small "keys" that the horse can move with its tongue. Mouthpieces may be smooth, wire-wrapped or otherwise roughened, or of twisted wire or metal.