LITS Logic Puzzle

LITS (nuruomino) is a shading puzzle on a board of regions. You shade some cells and leave the rest empty. Each region holds exactly four shaded cells, and those four cells form one tetromino. The shaded cells of the whole board join into one group. You can play 5×5 easy and medium, 6×6 medium, and 8×8 easy and medium. Every puzzle here has exactly one shaded set.

Rules

The board is an n×n grid divided into regions. Thick lines are the region borders. Thin lines are the cells inside a region. A region is a group of cells that share sides.

Shade exactly four cells in each region. Those four cells must form one of four shapes. An L is three cells in a line with one cell beside an end, and the J turn is the same shape. An I is four cells in a straight line. A T is three cells in a line with one cell beside the middle. An S is a zigzag of four cells, and the Z turn is the same shape. A 2×2 square is not allowed.

Two shaded pieces of the same shape must not share a side. Pieces of different shapes may share a side. Every shaded cell must connect to the other shaded cells through shared sides, so the shaded cells form one group. A fully shaded 2×2 block is not allowed, even when the four cells sit in more than one region.

A dot is a mark for a cell you already know is empty. Dots are optional. The puzzle does not ask you to dot every empty cell.

The puzzle is solved when the shaded cells are that unique set. Empty cells may still be blank. You do not have to place a dot on every empty cell. Finished pieces are not labelled with a letter.

Controls

Choose Shade or Dot under the board. Tap a cell, or drag across cells, to place that mark. Tap a cell that already has the mark to clear it. A cell that has the other mark takes the new one when you tap it. A drag follows the first cell: it places the mark on empty cells along the path, or it clears when that cell already has the mark. With a mouse, right-click places a dot, or clears a dot that is already there.

On a keyboard, the arrow keys move around the grid. Space or Enter places the selected mark, or clears it when the cell already has that mark. S or 1 shades the cell. X, D, E, or 2 places a dot. M switches Shade and Dot. Backspace, Delete, or 0 clears the cell.

The size control lists 5×5, 6×6, and 8×8. The difficulty control lists the bands that size ships. 5×5 and 8×8 list Easy and Medium. 6×6 lists Medium only. New game builds the next grid in a worker, and the board shows Generating while it waits.

A cell turns coral when a region has too many shaded cells, too few cells left to reach four, four shaded cells that are not a tetromino, a 2×2 block is shaded, two pieces of the same shape share a side, or the shaded cells are split into more than one group. Press Hint for one step the rules already force, with a sentence saying why. Press Check when you want the board checked against the rules. Undo and Redo step through your moves, and Reset clears the board. When the shaded cells are the unique set, the board turns teal.

Techniques

Easy grids follow from one region at a time, plus the rule that a 2×2 block cannot be fully shaded:

• No square: three cells of a 2×2 are shaded, so the last cell stays empty.
• Overlap: every way to place this region’s tetromino covers these cells, or none of them do.
• Beside a neighbour: only some placements still fit next to the neighbouring regions.

Medium grids also keep the shaded cells in one group. A cell that every remaining placement needs is shaded. A cell that none of them use stays empty.

Beginner tips

Start with a 5×5 easy grid. Look for a region that can hold only one shape, and shade the cells that every placement of that shape covers.

When three cells of a 2×2 block are shaded, dot the fourth before you look for the next piece. Then check that two pieces of the same shape do not share a side.

The daily puzzle is 6×6 medium. Move to 8×8 once the regions on 5×5 feel natural.