The squeaky board is a trap that wakes nearby sleeping monsters.
Squeaky boards can be generated on most levels. They are also guaranteed on the final level of the Samurai quest, the final level of the Valkyrie quest, certain versions of Medusa's Island, the Fake Wizard's Tower and the real Wizard's Tower.
Each squeaky board has an associated note: it may be any of the following:
- "C note"
- "D flat"
- "D note"
- "E flat"
- "E note"
- "F note"
- "F sharp"
- "G note"
- "G sharp"
- "A note"
- "B flat"
- "B note"
No two boards on the same level will squeak with the same note unless there are at least 13 squeaky boards on the level.
A squeaky board is triggered if you (or a monster) steps on it without flying or levitating. When triggered, the board wakes up nearby monsters. 
For you, "nearby" means within a distance of √20 × XL. (For example, level 5 gives a 10-square radius, level 10 gives 14, and level 15 gives 17.)
For monsters, "nearby" means that the distance is less than √40 (a 6-square radius).
Squeaky boards can be untrapped using the #untrap command. You must select a potion of oil (which will be consumed) or a charged can of grease (one charge will be consumed).
Since squeaky boards do not directly cause damage, they are often harmless. However stepping on a squeaky board can potentially wake up the monsters in a treasure zoo or similar special rooms, causing them to seek you out before you are ready to fight them. On the special levels where they are guaranteed, squeaky boards can often wake up enemies such as Medusa, the Wizard of Yendor and Ashikaga Takauji (who will wield the Tsurugi of Muramasa and possibly bisect you if he wakes up).
The unique note of a squeaky board can potentially help you pinpoint a monster's location.
Untrapping a squeaky board is probably not worth it, as there are better uses for both oil and grease.
Squeaky boards were added in NetHack 1.3d.
The ability to distinguish boards by note was added in 2006. It was inspired by the encyclopedia entry, which was included much earlier. The feature existed in the leaked development version, where it was listed as "multiple squeaks for squeaky boards" in the changelog. The feature was officially released in NetHack 3.6.0.
- You notice a loose board below you.
- You were flying or levitating over a squeaky board, and did not trigger it.
- You notice a crease in the linoleum.
- As above, but you were hallucinating.
- A board beneath you squeaks <note> loudly.
- You stepped on a squeaky board and triggered it, possibly waking nearby monsters.
- A board beneath you vibrates.
- As above, but you were deaf.
- A board beneath <monster> squeaks <note> loudly.
- A monster stepped on a squeaky board and triggered it.
- <monster> stops momentarily and appears to cringe.
- As above, but you were deaf.
- You hear a <note> squeak loudly.
- You hear a <note> squeak in the distance.
- A monster stepped on a squeaky board and triggered it, but it was outside your line of sight.
A floorboard creaked. Galder had spent many hours tuning them, always a wise precaution with an ambitious assistant who walked like a cat.
D flat. That meant he was just to the right of the door. "Ah, Trymon," he said, without turning, and noted with some satisfaction the faint indrawing of breath behind him. "Good of you to come. Shut the door, will you?"
- ↑ src/trap.c in NetHack 3.6.6, line 1566
- ↑ src/trap.c in NetHack 3.6.6, line 356
- ↑ src/mon.c in NetHack 3.6.6, line 1054: Effect on player
- ↑ src/mon.c in NetHack 3.6.6, line 2263: Effect on monsters
- ↑ src/mon.c in NetHack 3.6.6, line 3046
- ↑ src/mon.c in NetHack 3.6.6, line 3053
- ↑ https://github.com/NetHack/NetHack/commit/ab947c6b651df8f4dd7fbd79bb3d4dedd8a405c1