Topside can be used as a dual front and rear light but also as a front or rear light alone. The mounts provided all fitment to almost all helmets. At 68 grams it's hardly noticeable when worn and with up to 18 hours run time (flashing) it'll last a good chunk of an average week's riding. Solid, waterproof, USB rechargeable, clever features and light, this is a fabulous light for all cyclists.