Apple Modern Buckle Band - 38mm - Black - Small What looks like a solid buckle is actually a two-piece magnetic closure thats delightfully simple to secure. We also added an inner layer of Vectran weave for strength and stretch resistance. Its the same material NASA used to make the landing airbags for the Mars Rover spacecraft. Small fits 135-150mm wrists. Medium fits 145-165mm wrists. Large fits 160-180mm wrists.