What is the purpose of different snaffle ring styles — O-ring, D-ring, full cheek — and when is each appropriate?

The ring style of a snaffle bit affects how the bit moves in the horse's mouth, how it contacts the corners of the lips, and how clearly it signals the direction of lateral pressure — all of which affect how clearly the horse can read the rein communication. The O-ring — loose ring — snaffle has a ring that rotates freely through the end of the mouthpiece. This free rotation means the bit can move slightly in the horse's mouth, which some horses find comfortable and which prevents the horse from leaning on a fixed contact. The loose ring also means there is a small lag between rein movement and mouthpiece rotation — the ring slides slightly before the mouthpiece moves — which gives the horse a brief warning before pressure fully engages. Anderson and Parelli both use loose ring snaffles for starting horses specifically because of this slight lag and the comfort of the free movement. The D-ring snaffle has a fixed ring that does not rotate, meaning the mouthpiece moves immediately with rein movement. This produces a more direct, immediate signal than the O-ring, which some horses respond to more clearly and some find more abrupt. D-rings also have a slight directional stabilizing effect because the straight side of the D presses against the horse's lip when lateral pressure is applied, helping signal turning direction. The full cheek snaffle has long vertical extensions above and below the ring that prevent the bit from being pulled through the horse's mouth when strong lateral pressure is applied, and that provide additional directional signaling by pressing against the lip and cheek. Full cheeks are often used for young horses or horses with training holes in their lateral responsiveness, as the cheek piece makes the direction of pressure unambiguous. Parelli uses full cheek snaffles in his early foundation work for exactly this clarity of signal.