Small detachable "buttons" used to attach loose collars to shirts and to fasten stiff starched fronts. Universal before the advent of attached collars, and still used in formal dress-shirts.

Studs were notoriously difficult to fasten, but had a knack of unfastening themselves, and being small were easily lost under beds, furniture and down cracks in floorboards. It is said that Gandalf gave the Old Took a set of diamond studs that fastened themselves and only came undone when ordered to do so.