As a quick guide for those new to western, the following maybe helpful regarding bosals and sidepulls.
The bosal (pronounced “bo-SAL,” not “BO-sul”) is the classic hackamore and is seen primarily in in western-style riding and derived from the Spanish tradition of the Vaquero. It consists of a fairly stiff rawhide noseband with reins attached to a large knot or button at the base – the bosal. The reins are made from a specially tied length of rope called a mecate (may-CAH-tay), which is tied in a specific manner to both adjust the size of the bosal, and to make a looped rein with an extra length of rope that can be used as a lead rope. In the Texas tradition, where the bosal sets low on the horse’s face, and on very green horses in both the California (Vaquero) and Texas traditions, a specialized rope throatlatch called a fiador (FEE-a-dor) is added, running over the poll to the bosal, attached to the hackamore by a browband.
The fiador keeps a heavy bosal properly balanced on the horse’s head without rubbing or putting excess pressure on the nose. However, it also limits the action of the bosal, and thus is removed once the horse is comfortable under saddle. The bosal acts on the horse’s nose and jaw, and is most commonly used to start young horses under saddle in the Vaquero tradition of the “California style” cowboy. The bosal is a very sophisticated and versatile style of hackamore.
Bosals come in varying diameters and weights, allowing a more skilled horse to “graduate” into ever lighter equipment. Once a young horse is solidly trained with a bosal, a bit is added and the horse is gradually shifted from the hackamore to a bit. While designed to be gentle, Bosals are equipment intended for use by experienced trainers and should not be used by beginners, as they can be harsh in the wrong hands.
The sidepull is a modern design inspired by the bosal. It is a heavy noseband with side rings that attach the reins on either side of the nose, allowing very direct pressure to be applied from side to side. The noseband is made of leather, rawhide, or rope with a leather or synthetic strap under the jaw, held on by a leather or synthetic headstall. Sidepulls are primarily used to start young horses or on horses that cannot carry a bit. While severity can be increased by using harder or thinner rope, a sidepull lacks the sophistication of the bosal.
The primary advantage of a sidepull over the bosal is that it gives stronger direct lateral commands and is a bit easier for an unsophisticated rider to use. Once a horse understands basic commands, however, the trainer needs to shift to either a bosal or to a snaffle bit to further refine the horse’s training. If made of soft materials, a sidepull is also a good bridle for beginners to use, so that they don’t injure their horse’s mouth as they learn the rein aids.
English riders sometimes use a jumping cavesson, which is a type of hackamore that consists of a heavy leather nosepiece (usually with a cable inside) with rings on the sides for reins, similar to a sidepull, but more closely fitting and able to transmit more subtle commands. A jumping cavesson is put on a standard English-style headstall and often is indistinguishable at a distance from a standard bridle. It is often used on horses who cannot tolerate a bit or who have mouth or tongue injuries.