Birkenstock Boston Unisex Clog in Natural Dark Brown Classic Oiled Leather Leather Imported Rubber sole 'Boston' Classic Oiled Dark Brown Leather Clog. Upper material: Oiled Leather - thick yet supple leather is breathable, durable and comfortable. Softens with wear. Footbed: The classic Birkenstock footbed - features pronounced arch support, a deep heel cup, and a roomy toe box. The footbed conforms to the shape of your foot. Sole material: EVA - flexible, lightweight, durable and shock absorbing. Birkenstock sizing information: The letter "M" indicates "Medium/Regular/Normal Width Footbed". The letter "N" indicates "Narrow Width Footbed".