From Discworld & Terry Pratchett Wiki
Mine sign is a dwarfish cultural phenomenon arising out of the often cramped conditions of living in mines and habitually carrying axes all the time. Getting along becomes incredibly important, lest someone become tempted to put the non-mining and non-prospecting blade of their axe to use, hence the habit of drawing unattributed symbols on the walls, or anything else to hand, as a kind of system of gossip. While plenty of these signs, such as the Long Dark, are a part of the ordinary dwarfish language, but some of them, such as the Summoning Dark are "Unbelievably Mystical," what with how the dwarves revere the written word.
Known mine signs
- The Long Dark: A circle with a horizontal line through it. The most common symbol, it merely signifies a mine, and is a part of the ordinary dwarfish language. (Eerily similar to the London Underground logo: all the more so considering the plans Vetinari has for the future of the city.)
- The Following Dark: A circle with two diagonal lines through it. Means things are getting really bad: "We await what follows with dread." Can also be translated as "Repent, ye sinners!" (Also similar to "Urban Clearway - No Parking" GB roadsigns)
- The Closing Dark
- The Opening Dark
- The Breathing Dark: Used rarely.
- The Speaking Dark: Considered fine.
- The Catching Dark: Considered fine.
- The Secret Dark: Considered fine.
- The Calling Dark: Considered to be very dangerous.
- The Waiting Dark: The dark that waits to fill new holes.
- The Summoning Dark: Like a circled eye with a tail. This is the most dangerous sign of all, and when drawn it's considered safest to keep it exposed to bright light at all times; there is some kind of quasidemonic entity with which it is associated, which can be called down as vengeance by a sufficiently determined, and presumably dying, individual. Even when this happens, it cannot take action on its own but must take control of and work through a person.
- The Guarding Dark: Also known as the Watchman. The only entity that can master, defeat and contain the brooding vengeful violence of the Summoning Dark. The Guarding Dark appears to have no known minesign. But it is clear that where the Summoning Dark goes, the Guarding Dark follows, at its own steady inexorable pace. The only known example of this was in the extraordinary head of Sam Vimes whose rage and desire to be Lawful means that he watches himself, and governs his own inner beast. Wherever the Watchman goes inside the dark confines of Vimes's mind, he brings his lantern, dispelling the night. More here:-