Rein aids are the various ways a rider can apply rein contact to communicate specific requests to the horse through the bit, and understanding the distinctions between them is fundamental to developing educated hands and a well-schooled horse. Each type of rein aid applies pressure to a different part of the horse's mouth or neck and produces a different response — conflating them or using them interchangeably produces a horse that is confused, stiff, and ultimately heavy in the hand.
The direct rein, also called the opening rein, is the most basic: the rider's hand moves sideways and slightly back, creating a direct line of communication from the rider's hand through the rein to the bit and onto the horse's mouth on that side. It asks the horse to move its nose and forehand in the direction the rein leads, like opening a door. It is the primary steering rein for beginning horses and riders.
The indirect rein, or neck rein, applies pressure against the horse's neck rather than directly to the mouth. The rein crosses slightly over the withers or neck and suggests lateral movement away from the pressure. It is the primary steering tool in Western riding and in advanced English riding, where leg and weight aids provide the primary directional guidance and the reins merely refine the direction.
The bearing rein or neck rein in its Western form acts on the horse's neck without mouth pressure when the horse is trained to it — the horse has learned to move away from rein pressure on its neck without the bit needing to engage at all. The pulley rein, the opening rein, the counter rein, and various combinations and refinements of these basics are all built from these foundational types.