The brake caliper is the assembly which houses the brake pads and brake pistons. The brake pistons are usually made of aluminum or chrome-plated iron. There are two types of brake calipers: floating or fixed. A fixed brake caliper does not move relative to the disc. It uses one or more pairs of pistons to clamp from each side of the disc, and is more complex and expensive than a floating caliper. The caliper holds the brake pads. It straddles the brake rotor and uses hydraulic pressure from the brake lines, along with internal pistons, to force the brake pads against the rotor.