Abstract: A fastener, adapted to attach a cord or a chain to a sidepiece of a pair of eyeglasses, has a cylindrical tightening ring and an elastic loop having a cross section with a flat side and a curved side. The inner diameter of the tightening ring is smaller than the length of the flat side of the cross section of the elastic loop. When the elastic loop is passed through the tightening ring, the flat sides of two cross sections of the elastic loop face each other and the curved sides of the two cross sections face the inner surface of the tightening ring to secure the tightening ring to the elastic loop. When the fastener is attached to the sidepiece, the flat side of the cross section of the elastic loop faces the sidepiece to secure the elastic loop to the sidepiece.