fter the initial familiarization with the reins, a rider should be taught to start using them as aids. The reins are primarily used in the process of gymnastisizing or bending the horse laterally as well as vertically. To be able to do this, a coordination of all physical aids is necessary. The ability to flex and bend correctly is one of the hallmarks of a truly supple ("durchlassiges") horse. This flexion has to go from the poll over the neck, back and rib area to the dock in the same degree of bend. Even in the case of experienced Grand Prix horses, this gymnasticizing process is a continuous part of the daily work.

The hallmark of finely tuned cooperation between the mouth of the horse and the hand of the rider is that the pressure of the ring finger on the rein be all that is required to make the horse aware of the rider's wishes.

Although use of the rein aids in turns or on bent lines (for example, while riding through corners and circles) to many riders seems to be an easy concept relative to the other aids, practice shows a very different picture. The timing of the rein aids must be as precise as the seat and leg aids, and the degree of this precision determines not only whether the horse will have a soft or hard mouth. This has an impact on the horse's entire body and determines whether the horse will end up a finely tuned, sensitive and expressive animal or a dull school horse. Rein pressure must be precisely timed to be in the instant when the horse can shift his weight and find his balance, otherwise the horse will merely stiffen in response to the rein pressure.

The reins must be used both together and separately. The outside rein fulfills a very important function: it controls the amount of bend. Therefore, it has a governing function. It prevents the horse from merely bending his neck in response to the pressure of the inside rein and leg, thus falling out over the outside shoulder. The inside rein initiates the bend. The degree and amount of bend has to be continuously adjusted to every situation and to each individual horse. In short, the inside gives the command to bend and the outside rein governs and controls the amount of bend.

A further role of the outside rein is the collection function, for example at the strike off and in maintaining of the canter. The collection function of the outside rein makes the horse become more compressed in his entire body, and this is a key component of any higher dressage exercises. When the horse is properly elasticized by the rein aids, he becomes capable of bending in the rib area and by doing so can move his center of gravity over his inside pair of legs to an increasing degree. This leads us back to straightening the horse, since only a horse that is straight is able to collect to the highest degree. In short, one could say that all higher exercises are initiated with the outside rein.

Both hands must be held by a breathing hand. A breathing hand is never held completely still, and gives direction by opening and closing depending on the needs of the situation. It is important that the hand never block or interfere with the natural movement of the head and neck. By the same token, softness can be as detrimental as inappropriate forcefulness if it is allowed at the wrong instant. Therefore, in giving the rein aids, the ability of the rider to know precisely when to give them is crucial to the success or failure of training the horse.