Side reins are a type of rein used primarily when longeing a horse, running from the bit of the bridle to the saddle or surcingle. As a horse training tool, they encourage flexion and softness in the horse's mouth. For longe line work with a rider up who does not carry ordinary riding reins, they help calm and settle the animal. However, they are a tool best used by experienced handlers; used improperly they may unduly restrict the horse's movement or cause an accident.

Side reins are made of leather or webbing, sometimes with added elastic, and have several rings or holes for buckles along their length. They are easily adjusted. Some designs have adjustable buckles and attach to the bit with a snap, other designs run through the bit ring, then fold back on themselves and snap to their own rings.

Side reins may be completely of solid material, or they may have an elastic or rubber ring insert. Each design has its advantages and disadvantages. Designs with elastic have more "give" to them, which is useful for sensitive horses or horses that throw their heads. Elastic inserts must be used with caution, however, as some horses learn to lean on them. Solid side reins are the older, classical design, give the horse a more solid contact to work into and discourage leaning, but must be adjusted with greater care because their lack of give may upset a sensitive horse and, particularly if too tight, may provoke rearing, headshaking and even panic in some animals. Side reins with a rubber donut provide some give, although not as much as elastic and so discourage leaning. However, they are heavier and are prone to bounce when the horse trots or canters, which does not provide as steady a contact as the solid or elastic-insert side reins.

Elasticized side reins are often used with training young horses prior to being ridden. They help accustom a horse to the feel of pressure on the bit, and reward the horse when it gives or flexes to bit pressure. Solid side reins are usually used for more advanced horses. They give the horse something to take contact with, encourage balance and correct head carriage, help a horse develop self-carriage, and help stop a horse from over-bending in the neck.

Side reins are also used with in-hand showing.

Side reins may be attached from the bit rings to the surcingle rings, or from the bit to the buckles of the girth of an English saddle or cinch rings of a western saddle.

Side reins are adjusted longer for less-experienced horses, and gradually shortened and raised higher (from point of shoulder up to the point of hip) as a horse becomes better trained. Side reins should never be so short that the horse's head is pulled behind the vertical. For green horses, the side reins should be adjusted so that the horse's head is approximately 4 inches in front of the vertical and the side reins are attached at a point level with the point of the shoulder.

As the horse becomes more advanced and more physically developed, the side reins may be shortened so the head is nearly vertical to the ground. Side reins should not pull the horse in—they do not create collection. Rather, a properly longed horse will collect himself, and the shorter side reins will be the correct length for him to keep a contact with the bit. Short side reins should not be used for long periods of time.