The Madrid is the oldest member of the family of models, making it a veritable classic. Launched as a “gymnastics sandal” half a century ago, this simple single-strap model has gone on to become a firm fashion favorite. The Big Buckle look offers yet another fashionable element to an already classic sandal. Birkenstock uses sturdy leather with a thickness of 2.8 to 3.2 millimeters as upper material.