A lip strap is a piece of horse tack

Horse tack

Tack is a term used to describe any of the various equipment and accessories worn by horses in the course of their use as domesticated animals. Saddles, stirrups, bridles, halters, reins, bits, harnesses, martingales, and breastplates are all forms of horse tack...

 used sometimes on some types of curb bit

Curb bit

A curb bit is a type of bit used for riding horses that uses lever action. It includes the pelham bit and the Weymouth curb along with the traditional "curb bit" used mainly by Western riders....

 and Pelham bit

Pelham bit

A pelham bit is a type of bit used when riding a horse. It has elements of both a curb bit and a snaffle bit. In this respect a pelham bit functions similar to a double bridle, and like a double bridle it normally has "double" reins: a set of curb reins and a set of snaffle reins. Because it has...

s. The lip strap runs between the bit shank lever arms (see Lever#Curb bit) and passes through a special center ring on a curb chain

Curb chain

A curb chain, or curb strap, is a piece of horse tack used on any type of curb bit. It is a flat linked chain or strap that runs under the chin groove of the horse, between the bit shank purchase arms. It has a buckle or hook attachment and often has a "fly link" in the middle to apply a lip strap...

(not all curb chains are designed for use with a lip strap). The lip strap attaches to rings at midpoint of the lever arms and buckles on the near side.

The lip strap helps keep a "mouthy" horse from mouthing or "lipping" the shank. It also helps prevent the curb chain from unfastening or otherwise moving too much.

The leather attachments that are sometimes used to connect the rings of snaffle bit are also occasionally known as lip straps.