A dog collar is a piece of material put around the neck of a dog. A collar may be used for restraint, identification, fashion, or protection. Identification tags and medical information are often placed on dog collars. Collars are often used in conjunction with a leash for restraining a dog. Collars can be traumatic to the trachea if the dog pulls against the restraint of the leash, causing severe pressure to the neck. Use of a harness instead of a collar may be beneficial for dogs prone to tracheitis or those with a collapsed trachea. Conversely, dog breeds with slender necks or smaller heads may easily slip out of collars that are too loose. This can be avoided by using a martingale dog collar which tightens to distribute pressure around the neck when training the dog not to pull.