Material: Rubber; Your strap loop usually broken earily than your main strap; They can fit any watch bands; Size 18mm : Inner Size,1.8x0.8cm/0.71x0.31inch;Outer Size,1.1x2.2cm/0.43x0.87inch Size 20mm : Inner Size,2x0.7cm/0.79x0.28inch;Outer Size,1x2.3cm/0.39x0.91inch