In these applications the guide line is generally more for efficiency than for safety, but it may also help the divers stay away from potentially hazardous areas.

A dive reel comprises a spool with a winding knob, which rotates on an axle, attached to a frame, with a handle to hold the assembly in position while in use.

Other accessories which may be present (usually not all on the same reel) include: Reels may be made from a wide variety of materials, but near neutral buoyancy and resistance to impact damage are desirable features, which are easiest to achieve in engineering polymers such as nylon, acetal (delrin) and polyethylene.

Open reels allow easy access to free jams caused by overwinds or line getting caught between spool and handle.

They are simply a pair of circular flanges with a hole in the middle, connected by a tubular hub, which is suitably sized to use a finger as an axle when unrolling the line.

The small, compact size, and low cost make them useful for various purposes where long line is not required.

[4] Spools may be made from any material that is strong enough and suitable for underwater service, but engineering thermoplastics are most common.

A simple type is an H-shaped piece of wood, plastic or stainless steel used to manually wrap a moderate length line.

They are often specifically designed for laying and recovering line at swimming speeds, and may differ from reels used for other purposes such as deploying a DSMB.

They have negative buoyancy and enough line to reach the surface from the planned deployment depth, and are often made of injection moulded plastic and sold as a set with the DSMB.

Larger versions which can carry more line and are suitable for use from deeper depths are more likely to be machined from aluminium and are both more ergonomically shaped and more expensive.

General purpose reels used by open water divers usually feature a ratchet mechanism which allows rapid deployment of DSMBs, and secure recovery of line, using the spring-loaded ratchet to prevent unintended unrolling during ascent, but allowing deployed length to be increased rapidly under tension by disengaging the pawl, which is usually operated by a thumb lever or finger trigger.

Ratchet reels are occasionally fitted with a drag mechanism to allow controlled line tension when laying guide line, and a ratchet release retainer, but these features add complexity and cost and potential failure points, and are not needed for open water use.

One important reason to be adequately trained before cave diving is that incorrect marking can confuse and fatally endanger not only oneself, but also other divers.

Silt screws are pegs which are inserted into soft bottom sediments to tie off the guide line when there are no suitable natural formations.

A common style of silt screw is a length of rigid PVC tube cut to a point at one end, with a notch at the other to secure a wrap.

The principle of a continuous guide line between the penetration diver and open water is central to cave and wreck diving safety.

[2] The line should be laid so that it can be followed in the worst possible visibility, and allow for other emergency procedures such as gas sharing at the time.

A lock also puts the incoming and continuing parts of the line in contact, which makes it much easier to follow by feel.

[2] The security of both placements and tie-offs depends on the detail of the place where they are made, and some may be disrupted by a pull on the line in the wrong direction.

More frequent placements and tie-offs are likely to be used on a permanent line which does not have to be retrieved on the way back, and which must withstand use by many divers over a long period.

[2] The line would ideally allow divers to see or feel it while swimming comfortably without coming into contact with or being obstructed by, the bottom, sides or ceiling.

[2] The secondary tie-off is made just inside the overhead, where the route to the exit is obvious even if visibility is lost.

In an emergency, the reel would be left at a tie-off and the divers would exit without recovering the line, as this would save time.

[2][9] It can be useful to build up a mental model of the route, even to make notes and sketches indicating major landmarks and changes of direction.

This helps to reduce disorientation on return along the line, when the surroundings may look very unfamiliar because they are observed from a different direction.

It is more conservative gas management to do any work on the outward leg, and return directly, unless specifically planned otherwise.