Slip collars (also called choke chains, slip chains, or choke collars) are a length of chain or nylon rope with rings at either end such that the collar can be formed into a loop around the top of the dog's neck, just behind the ears. The ring which connects to the leash goes over the back of the dog's neck, not under. When the leash is attached to the dead ring the collar does not constrict on the dog's neck. When the leash is attached to the live ring the chain slips (adjusts) tighter when pulled and slips looser when tension is released. A quick gentle jerk with an immediate release, called a leash pop, snap, or correction, is used to alert the dog that it has done something undesirable.
The reference to choking is due to the fact that slip chains typically have no limit to how far they can constrict on the dog's neck (versus limited slip collars which do have a limit to how far they can constrict). This name for the collar is deceiving however, as proper use of this training device not involve choking (strangling) the dog. Even so, it is important to note that improper use of this collar can result in injury and even death of the dog, for the aforementioned reason.
Prong collars (also called pinch collars) are a series of chain links with blunted open ends turned towards the dog's neck so that, when the collar is tightened, it pinches the naturally loose skin around the dog's neck. The design of the prong collar is such that it has a limited circumference, unlike slip collars, which do not have a limit on how far they can constrict on a dog's neck. Similar to the martingale collar, any pressure from the collar to the dog's neck is spread out over a larger area than with most buckle collars and with all choke chains.
Prong collars can also be turned inside out (with the prongs facing away from the dog's skin), to function like a martingale. Rubber tips are occasionally placed on the ends of the prongs to protect against scratching or puncturing the skin, though it is difficult to actually puncture the skin using this type of collar, because of this spreading effect, the limited constriction, and the already blunted tips. Like the slip collar, the prong collar should be placed high on the dog's neck, just behind the ears.
Some dogs can free themselves from prong collars by shaking their head so that the links pop out, so some trainers have come to use a second collar (usually a flat buckle collar) in addition to the prong collar.
Martingale collars are recommended for Sighthounds because their heads are smaller than their necks and they can often slip out of standard collars. They can, however, be used for any breed of dog. Their no-slip feature has made them a safety standard at many kennels and animal shelters. A martingale collar has 2 loops; the smaller loop is the "control loop" that tightens the larger loop when pulled to prevent dogs from slipping out of the collar. Similar to a prong collar, the martingale has limited constriction on the dog's neck and applies even pressure.
Head halters, sold under the brand names Halti or Gentle Leader or Snoot Loop, are similar in design to a halter for a horse. This muzzle-like device fastens around the back of the neck and over the top of the muzzle, giving more control over a dog's direction and the intensity of pulling on a leash than collars that fit strictly around the neck. Pressure on this type of collar pulls the dog's head towards the handler.
Opponents of the head halter say that most dogs find it unnatural and uncomfortable. If the collar is too tight, it may dig too deeply into the skin or the strap around the muzzle may push into the dog's eyes. Injury can result from improper use of the head halter; if a dog is jerked suddenly by the leash attached to the head halter, the dog's neck is pulled sharply to the side, which might result in neck injury.