A dog collar is a piece of material put around the neck of a dog. A collar may be used for restraint, identification, fashion, protection, or training. 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. Any style of dog collar must be properly fitted to ensure safety and collars should not be worn when the dog is unattended.