Downline (diving)

In underwater diving, a downline is a piece of substantial cordage running from a point at the surface to the underwater workplace, and kept under some tension. It can be used as a guideline for divers descending or ascending,^[1] for depth control in blue-water diving,^[2] and as a guide for transfer of tools and equipment between surface and diver by sliding them along the downline at the end of a messenger line.^[1] A shotline is a special case of downline which uses a heavy weight at the bottom and a float at the top. A jackstay is a more lateral equivalent, that commonly follows a surface, and will not usually allow materials transfer without a messenger line from the destination end.