Shaped mouth is a catchall term that generally applies to bits labeled as “ergonomic” or “comfort.” Such bits are typically mullen mouths with a wavy shape in the mouthpiece (in which case they are likely made of a polyurethane material), or jointed mouths with curved arms.  Many brands carry extensive lines of bits that have shaped mouthpieces, such as the Dynamic RS bits from Herm Sprenger, the JP bits by Korsteel, and most of the Mylar bits.

The wavy shaped mullen mouths are not particularly different from a regular mullen mouth, except that the curves may vary the pressure points on the horse’s tongue slightly and give some horses more incentive to chew the bit.  The principle of the shaped arm pieces is to curve the mouthpiece such that it fits ergonomically over the horse’s tongue, thereby allowing more room for the tongue to move, as well as prevent the bit from pressing into the roof of the mouth.  The tradeoff in having curved arms is that there is potentially more pressure put over the bars of the mouth, depending on the shape, which may be either advantageous or disadvantageous depending on the horse.  Also, how the shape of an individual horse’s tongue and palate interact with the particular shape of the bit is highly variable, so what may be ergonomic for one horse may cause pinching and interference for another.  As with all bits, trial and error is generally the best way to find what is most comfortable for any individual horse.