Disc brake adapters - use different brake calipers and discs
Disc brake adapters mount between frame or fork and the brake calliper or the brake disc. In general, they have two functions. They enable you to mount different types of brake calipers: the three standards are IS (international standard), post mount and flat mount. There's also the so-called brake disc adapter, which allows you to attach 6-hole discs to hubs designed for center lock discs, and vice versa.