What are collar stays?
Removable slats of plastic or metal which are attached under a shirt's collar. They reinforce both ends of the collar to keep it neat and taut. The size of the collar stay varies by clothing brand and type of shirt.
History of collar stays in (men's) fashion
At the time of whale hunting (from the 17th to the 20th century), pieces of baleen from a whale's beard were used to hold the shirt's collar in place. The beard is a sieve structure that sits in the mouth of a whale to filter plankton from the water. Whales' baleens are made of keratin, the same material your nails and hair are made of. In addition to shirts, suits, corsets, and shoulder straps were also reinforced with baleens.