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. Oiled leather can be recognized by its untreated surface. Birkenstock purposely does not treat surface grains and other characteristic features. Birkenstock uses sturdy leather with a thickness of 2.8 to 3.2 millimeters as upper material.