Riding aids are the cues a rider gives to a horse to communicate what they want the animal to do. Riding aids are broken into the natural aids and the artificial aids.
These are the aids which the rider possesses on their body, and should be used for the majority of the cues to the horse. Overuse of any aid can be detrimental to the training of the horse, but in general harsh or rough hands are considered the worst crime a rider can commit using the natural aids. The natural aids include:
It is important to remember that the aids are used in a spectrum, from very light to very powerful, depending on the response desired. A very sensitive horse may readily jump forward from light touch of the leg, while a dull horse may require a kick to get the same response. Additionally, an aid from canter to walk, for example, will use slightly more restraining aid on a particular horse than that horse would need going from canter to trot.
Positioning of the legs, seat, and hands are also used in a spectrum according to the individual horse and the response desired. For example, the aid for the canter depart may require the leg to be in a slightly different place than when it asks the horse to bend, or when it corrects hindquarters that are falling to the outside.
In all cases, good training aims for the horse to be responsive at the slightest cue, rather than requiring harsh aids to get the animal to respond. This is one reason why better-trained animals can be harder to ride, as they will respond to the slightest movement or shift in weight made by the rider. Therefore, they will take any mistake made by the rider as a cue to do something (such as a slight pinching of the legs as a cue to run forward, or a slight imbalance in the rider's seat as the cue to step sideways or speed up). Riders must therefore be sure that any perceived "disobediences" are not actually caused by their own doing.
Good training of the rider will aim to produce someone with an "independent seat", meaning someone who is able to give the aids independent of each other (without, for example, sitting forward while adding leg). The rider's first task is to learn to ride the horse without interfering: keeping a steady contact with the bit, sitting in a balanced, relaxed position that allows them to absorb the horse's movement, and keeping a steady, quiet leg that does not pinch, bounce, or push forward or back. Only then will the rider be able to really start to influence the horse in such a way to help it.
The leg, along with the seat, should be the main aid for the horse. It has a great deal of control over the horse's hindquarters, and is used to cue the horse to go forward, increase impulsion (power), step sideways, and correctly bend. It is the primary "driving aid" (cue to ask the horse to increase forwardness or power).