The continuous line from the cave entrance to your turn point — your way back when visibility is gone. Line arrows always point to the nearest exit.
The guideline is the only system that still works when visibility is completely gone: lights fail, and in true darkness your sense of direction almost always does too. It runs from a primary tie-off placed where a direct ascent would still be possible, laid under light tension — taut enough to find by touch, slack enough not to chafe on edges. Line arrows always point to the nearest exit, no matter which direction you came from. Personal cookies mark your own entry point, so on the way out you know for certain that you have passed it. The most expensive mistake is briefly letting go while switching hands or passing a teammate: put the second hand on first, then release the first. And one misunderstanding that keeps costing people — the line is not a handrail. You swim and merely touch it; pulling yourself along shifts the line and wrecks your own trim.