Line markers are used to provide information to divers following a distance line (aka guide line, penetration line, cave line) in overhead environments such as caves and wrecks. Markers are often personalized by the diver and sometimes further individualized with unique tactile modifications. They are attached to the guideline by pushing the continuous line into opposing slots on the marker. To securely fasten the marker, a wrap of the line must be added to the base of each slot as illustrated to prevent the marker from sliding along the line or falling off.

• Line Arrows are isosceles triangle shaped directional markers providing both a tactile and visual reference to indicate the path to the closest exit.
• Line Cookies are circular disk shaped non-directional markers to denote a specific location or circumstance on the line. They are often used to mark reference points, intersections, line gaps (i.e. jumps), or the presence of a diver on the line.
• Line Squares, also called referencing exit markers (REM), are rectangular shaped hybrid markers with a square area at the end opposite the slots to serve as a personal navigation tool but should be considered non-directional for anyone other than the diver placing the marker.

The usage of line markers may differ between localities, teams and individual divers because best practice recommendations have changed over time. The protocols for placement and removal are an important part of cave and wreck dive training, along with knowledge of local practices, because incorrect usage and misinterpretation of line markers are known to cause dangerous confusion and believed to be factors contributing to fatalities.