Play LITS Online
Place one L, I, T, or S tetromino in every region. All shaded cells must connect — no 2×2 pools and no same-type adjacency!
Created by Brian Hamilton
Tap cells to shade them — form one tetromino per region
How to play LITS
The grid is divided into regions. Shade exactly four connected cells in every region so that all four rules hold.
- Each region's four shaded cells must form an L, I, T or S tetromino — never the 2×2 square.
- All shaded cells on the board must form one connected group.
- No 2×2 block anywhere may be entirely shaded, even across region boundaries.
- Two pieces of the same shape may not share an edge. Rotations and mirror images count as the same shape; different shapes may touch freely.
- Every puzzle has exactly one solution.
Controls: tap a cell to shade it, tap again to clear it. Use Check to test your work and Solution to reveal the answer.
Four shapes, and the one that is missing
LITS gives you a grid divided into regions and asks for exactly four shaded cells in each, joined edge to edge. Four connected cells make a tetromino, and there are five of them — but only four are allowed. The name lists them: L, I, T and S. The fifth, the 2×2 square, is banned, and so is any 2×2 block of shading anywhere on the board, even one spanning several regions.
Rotations and mirror images count as the same shape. An L turned on its side is still an L, and so is its mirror. That matters for the rule that does the work.
Touching pieces are a gift, not a problem
The rule beginners misread is the one about neighbours. Two pieces may sit edge to edge — what is forbidden is two pieces of the same shape touching. On a finished board 4.7 pairs of pieces touch on average, and every one of those pairs is a pair of different shapes.
Read backwards, that is a deduction rather than an obstacle. If you know one region holds an L and it touches its neighbour, the neighbour is not an L. Since a region often admits only two or three shapes to begin with, one neighbour can settle it.
The trap is thinking the ban is about shapes that look alike. It is about shape type after rotation and reflection. Two L pieces pointing opposite ways are still both L, and still may not touch.
Start with the four-cell regions
A region of exactly four cells has no choice at all: its four cells are its tetromino. Shade the lot. 13% of regions are that size, so most boards hand you two or three pieces before you have reasoned about anything.
They are worth more than themselves. A placed piece fixes shape types for its neighbours, and because every shaded cell must end up connected to every other, a piece near the edge tells you which way the rest of the board has to reach.
| Shape | L | I | S | T |
|---|---|---|---|---|
| Share of pieces | 41% | 22% | 21% | 16% |
L turns up two and a half times as often as T, and that is not a quirk of these boards — it is arithmetic. Counting rotations and reflections, L has eight distinct orientations, S and T have four each, and I has just two. The shape with the most ways to sit in a region is the one that fits most often. When you are guessing which piece a region holds, guess L.
It also means only 34% of boards use all four shapes. Do not expect a full set; a board with no T on it is perfectly normal.
Roughly half the grid ends up shaded
Shading covers 52% of the board, and that is fixed by arithmetic rather than taste: every region contributes exactly four shaded cells, so the shaded share is four divided by the average region size. Regions here average 7.7 cells.
That ratio is why the difficulty setting changes the region sizes rather than anything else. Easy uses tighter regions and lands at 56% shaded, Hard uses looser ones and lands at 49% — fewer shaded cells, more room for a region to be ambiguous, more deduction needed.
Where solvers get stuck
Filling a region correctly and stranding the shading. Every shaded cell on the board must connect to every other, across region boundaries. A region can hold a perfectly legal tetromino and still be wrong because it leaves an island.
Forgetting the 2×2 rule crosses borders. Two pieces in neighbouring regions can each be legal and still form a forbidden square between them. Check the seams, not just the regions.
Treating a large region as free. Big regions look permissive, but they are constrained from outside — by what their neighbours have taken, by the connectivity requirement, and by the squares they must not complete. The largest region on these boards runs to twenty cells, and it is usually settled last rather than first.
Two things you can check about these puzzles
Every puzzle now has exactly one solution. This page never claimed otherwise, and its Check button has always tested the rules rather than comparing against a stored answer, so nothing was ever unwinnable. But a puzzle with many answers is barely a deduction, and that is what these were: measured against an exhaustive counter, a 7×7 board had a median of over 5,000 solutions, and not one region in 72 was forced.
The cause was the order of construction. The old generator placed the four-cell pieces first and grew regions around them, so every extra cell a region picked up added another place its piece could have gone. Boards are now built the other way round: partition the grid first, then move single cells across region boundaries, keeping each change only while the number of solutions does not rise, until it reaches exactly one. 84 boards across both sizes and all three difficulties were then re-checked against a separate counter written from the rules alone. All 84 were unique.
The grid sizes changed, and it is worth saying why. This page used to offer 10×10 and 12×12. Searching for a board that has exactly one solution is far harder than checking one, and at 10×10 it took about a minute per puzzle. LITS is genuinely difficult to generate — published collections are usually hand-built, and typically at 6×6 and 8×8. Two sizes that always work beat four that mostly do not.
LITS and its neighbours
LITS is Nikoli's, and the name is simply its four legal pieces. It shares the divide-and-shade frame with Nurikabe and Heyawake, but its regions do something theirs do not: they carry no number at all.
Nurikabe and Heyawake tell you how many cells a region gets. LITS tells you the count is always four and makes the shape the question instead — which is why its hardest rule is about what two neighbouring regions may not both be.
More shading puzzles
If you like deciding which cells to fill in, try these: