The hackamore is a bitless bridle that acts on the horse's nose bone. It consists of a noseband with a padded metal part, metal levers - the suits - and a chinstrap. The reins are buckled into the suits. As it has a strong effect on the nose bone, it can easily cause pain and serious injury if the rider is too rough or clumsy. The noseband must rest on the bridge of the horse's nose, not on the nasal bone underneath. The latter is very thin and might be broken if the worst happens. Also, if the noseband is too low, you can cut off your horse's air supply. For the same reason, the chinstrap should be placed as high as possible on the lower jaw, not directly on the chin.
To ensure that the bitless bridle does not fit too tightly, there should be a finger's width of space between the straps and the horse's body.