Collar stays also known as collar thingies, collar sticks, bones, knuckles, tabs in the US, in the UK, collar stiffeners, and in Eastern Canada collar stiffs. They are shirt accessories which are smooth, rigid strips of metal such as brass, stainless steel, mother of pearl, or plastic, rounded at one end and pointed at the other, inserted into specially made pockets on the underside of a 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 do not have this problem.
At a later stage, we will have metallic collar stays with a magnet for keeping collar straight and stiff. Collar stays are removed from shirts before dry cleaning or pressing, as they could damage the shirt in the process, and then replaced prior to wearing. Some shirts have stays which are sewn into the collar and are not removable (we have the Nylon collar stays of this kind with 0.25mm or 0.35mm thickness available).