The primary light would be optional on well illuminated recreational dives. Backup (reserve) lights are carried where they are unlikely to snag, and cause minimal drag, but can be reached and operated by one hand. Two are required for overhead diving. The storage position for backup lights is clipped to the chest D-rings and held against the harness by rubber bands, where they are tucked away and unlikely to snag, but remain easily accessible to both hands, and can be turned on before unclipping, so they can be easily found if dropped. Fresh non-rechargeable batteries should be installed before any dive where burn time of the backup light may be critical to safety, and burn time should allow exit from any point in the dive with a margin for problems. The “tow behind” scooter arrangement is preferred to the ride-on style scooter which creates higher drag as the scooter and diver stacked together present a greater cross sectional area to the water than when one follows the other, and they make it more difficult to tow another diver or additional gear. The tow behind scooter reduces these problems as the diver is towed in the wake of the scooter, the propeller is visible and in front of towed equipment, and they are more easily steered, particularly in tight spaces. Scooter tow lanyards run from the scooter handles and clip on to the front crotch strap D-ring when in use, to pull the diver by the D-ring, taking most of the load off the arms, and allowing control with one hand. The most efficient position for the scooter is at a relaxed arm's length in front, offset below so the propeller wash will not hit the diver. Depth gauges need to be viewable at all times, particularly during ascent and decompression. They are wrist-mounted on the right arm, to allow monitoring while buoyancy is being controlled using the left hand to operate the inflator hose and dump valves on the left. They should not be mounted on a bulky console and dragged behind the diver, as this could damage the environment on contact, or increase the risk of entanglement, and require retrieval of the console every time the depth is to be checked. The compass is mounted on the wrist, as the alternative mounting on a retractor or on a console are considered more likely to snag or drag on the bottom, thereby potentially damaging delicate structures and organisms. It is worn on the left to keep it away from the magnetic field of the scooter motor when a scooter is used.