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A Nikoli shading puzzle named after the four tetromino shapes it uses. Every bold-outlined region must hold exactly one L, I, T or S piece, and all the pieces together have to link into one chain — no 2×2 block of shading, and no two identical shapes touching across a border. Each 7×7 board has exactly one solution.
The board is a seven by seven grid divided into regions by bold lines. In every region you shade exactly four cells that touch each other side to side, so each region ends up holding one tetromino. Because of the rules below, only four shapes are ever possible: the L, the I, the T and the S, which is where the puzzle gets its name. Mirror images and rotations of a shape count as the same shape.
Three rules tie the regions together. First, all of the shaded cells on the whole board must form one connected group, so every piece has to touch at least one piece in a neighbouring region. Second, no two by two square may be completely shaded anywhere, even where two pieces meet. Third, two pieces of the same shape may never share an edge across a region border — an L may touch a T, but never another L.
Tap a cell to shade it. Tap it again to leave a small cross, a note for yourself that the cell is certainly empty, and tap once more to clear it. When a region holds a proper four cell piece its outline turns green and the piece takes on its shape's colour, which makes the same shape rule easy to see. Anything that breaks a rule, such as a fifth cell in a region or a shaded two by two square, turns red.
Every board has been checked by a solver to have exactly one solution, so it can always be finished by reasoning alone without guessing. The clock starts when the board appears, and your time to a complete, correct board is your score, so lower is better.
Start with the regions that are exactly four cells. They have no choice at all, so shade them straight away, then look at what their shape forbids next door. If a four cell region is an L, no L may sit against it, which often rules out most of the options in the neighbouring region at once. Small regions of five or six cells are the next easiest, because only a handful of tetrominoes fit inside them.
Look for cells that every possible piece in a region must use. In a region shaped like a fat T or a short corridor, the cells at the bottleneck are shaded no matter which piece you choose, and you can shade them long before you know the piece. The opposite trick works too: a corner cell that no four cell piece in its region can reach is certainly empty, and deserves a cross.
The two by two rule is the one people forget, and it is the most useful. Any time three cells of a square are shaded, the fourth must be empty, even if it belongs to another region. Where two regions meet at a corner, that single forced cross can decide which way a piece in the neighbouring region has to bend.
Connectivity decides the endgame. The whole shading is one chain, so a region hemmed in by empty cells must send its piece toward whichever neighbour it can still reach, and an isolated group of pieces tells you the missing link has to run through the one region that bridges them. When you are stuck, count the ways each unfinished region could connect to the rest — usually one of them has only a single road left.