The range of warning is indicated by question marks. In comparison, the range of unblind extrinsic telepathy is slightly smaller, and is indicated by flags.
Warning is a property conveyed by wearing a ring of warning or acquired by certain roles. It gives you advance warning of nearby monsters you can't see by representing them with a number from 1 to 5 based on their level. (1, 2, 3, 4, 5) The number is ML/4, where ML is the monster's level (though this will go no higher than 5, and 0 is not displayed). The glyphs displayed are described as an "unknown creature causing you <feeling>", where <feeling> is one of concern, anxiety, disquiet, alarm, or dread for warning levels one to five respectively.
In graphical user interfaces using the default tileset, such as the Windows client nethackw, the numbers are replaced with colored question marks: 
. Colors of the question marks correspond to the colors of the numbers given above.
Warning is considered a useful ring in ring strategy.
Besides the ring, some artifacts convey warning: the Orb of Fate and the Master Key of Thievery (and in SlashEM, The Candle of Eternal Flame) . A few roles also gain intrinsic warning when reaching a certain experience level: XL 7 as a monk and XL 15 as a caveman, healer, priest and wizard.
The exact criteria for warning to show you monster locations are:[1]
- The monster must not be peaceful.
- You must be under 10 tiles from it (i.e., it must be strictly within a ten-tile circle centered at you).[2]
- Its level must be greater than or equal to 4.
Messages[]
- "You feel sensitive!"
- You gained intrinsic warning through level gain
- "You feel less sensitive!"
- You lost intrinsic warning through level drain
Notes[]
- ↑ display.h, line 38
- ↑ display.h, line 44, but see also hack.h, line 288 and dist2 in hacklib.c.