Breastplate (tack)
A breastplate (used interchangeably with breastcollar, breaststrap and breastgirth) is a piece of riding equipment used on horses. Its purpose is to keep the saddle or harness from sliding back.
On riding horses, it is most helpful on horses with large shoulders and a flat ribcage. It is also a safety feature, especially on cross-country, should a rider's girth or billets break, as she will have enough time to stop the horse and dismount before the saddle slipped off the animal's back or underneath its belly. The breastplate is used on both English and Western saddles. When used in English riding, the hunting breastplate is made of thinner straps of leather, as is the western style used for horse shows. Working western horses in disciplines that involve work with cattle use a thicker, sturdier style.
Read more about Breastplate (tack): Harness, The Hunting or Stockman's Breastplate, Western Styles, Fitting, The Breastcollar/ Polo Breastplate, Breastgirth/loop Breastplate