These mirrors have been designed for rowing: Large Mirror (48mm diameter) clips onto baseball cap . The clip requires firm force to fit on brim to ensure it stays firmly in place - If it is too tight then warm clip in hot water before attaching - it will fit onto a cap rim with a thickness up to 5mm. Scratch proof – glass mirror. Robust – mirror protected with plastic surround. Easy to adjust. Suits scullers, coxswains in bow-loader boats (see what's coming up behind you) and steersmen in coxless boats. If you require a longer mirror, then you can order the 'Bicycle Mirror for Helmet Hat or Cap' (same price) - this is approx 5.5" between the centre of clip and centre of mirror, one inch longer than the sculling mirror which is approx 4.5".