Noseband

A noseband is the part of a horse's bridle that encircles the nose and jaw of the horse. In English riding, where the noseband is separately attached to its own headstall or crownpiece, held independently of the bit, it is often called a cavesson or caveson noseband. In other styles of riding, a simple noseband is sometimes attached directly to the same headstall as the bit.

A noseband may have been one of the first tools used by humans to domesticate and ride horses. The bit developed later.

The noseband was originally made of leather or rope. After the invention of the bit, the noseband was, in some cultures, demoted to a halter worn beneath the bridle that allowed the rider to remove the bit from the horse's mouth after work and leave a restraining halter on underneath, or to tie the horse by this halter, instead of by the bit, which could result in damage to the horse's mouth if it panicked. However, its ability to hold a horse's mouth shut over the bit was also recognized, as was its usefulness for attaching equipment such as a martingale, and so in some traditions it was sometimes left as a working part of a bridle. Still other cultures, such as that of Ancient Persia, developed the noseband as a tool for training young horses, called a hakma, and this training noseband evolved into modern equipment such as today's bosal-style hackamore and Longeing cavesson.

Today, the noseband has several uses:

There is a correlation between the sensitivity of a noseband and the amount of tension needed in the reins to obtain a response from the horse. In a 2011 study of horses being ridden in English riding equipment with the noseband in one of three adjacent adjustments, greater rein tension was needed to get a response from the horses when they had the looser adjustment. However, the study did not go on to examine the effects of no noseband at all or a very tight adjustment. Thus, nosebands may add some pressure to the nose when the reins are applied, depending on adjustment, style and the degree to which the horse resists the bit. With a soft leather noseband on a well-trained horse, the effect is minimal.

...
Wikipedia