Collar stays (sometimes known as collar sticks, bones, knuckles, tabs, and in the UK, collar stiffeners) are shirt accessories.
Collar stays are smooth, rigid strips of metal (such as brass, stainless steel, or sterling silver), horn, baleen, mother of pearl, or plastic, rounded at one end and pointed at the other, inserted into specially made pockets on the underside of a man's shirt collar to stabilize the collar's points. The stays ensure that the collar lies flat against the collarbone, looking crisp and remaining in the correct place. Often shirts come with plastic stays which may eventually need to be replaced if they bend; metal replacements don't have this problem.
Collar stays can be found in haberdashers, fabric- and sewing-supply stores and men's clothing stores.
Collar stays should be removed from shirts before dry cleaning or pressing, but then should always be put back by the dry cleaning company. Shirts that are press ironed with the collar stays are vulnerable to damage. Pressing with the collar stays in place results in a telltale impression of the collar stay in the fabric of the collar. Some shirts have stays which are sewn into the collar and aren't removable. This is the purpose of the small hole in the rounded end of the stay, the pointed end is often put in place against the corner of the collar and other end anchored by the small hole.