A rubber sewing machine belt. This belt will fit a wide range of domestic sewing machines. Usually, your sewing machine will have a visible motor (ie not a motor that is covered).
Obviously there is a certain amount of stretch in the belt. Howev...
A treadle belt for old Singer and Jones treadle sewing machines. The belt has to be cut to size and then a hole made in the end. Fit the staple through the hole and make sure it is bent back in to shape.