While it comes as a double-pass strap, the Convertible Rubber band can be converted to single pass with just a strap tool! Curious about FKM rubber? FKM is a class of fluorinated, carbon-based synthetic rubbers, commonly known as fluoroelastomer. FKM is characterized by high chemical stability and resistance to aging and weather. It is soft & flexible and will not attract dust and dirt like some less expensive, silicone rubber straps.