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Shade exactly four connected cells in each outlined region to form a tetromino, so that the following conditions are true: (1) All tetrominoes are connected into one large shape along their edges; (2) No 2×2 group of cells can be entirely shaded; (3) When two tetrominoes share an edge, they must not be of the same shape, regardless of rotations or reflections. (Not all four letters have to be present in the grid; for example, it is possible for your solution to not have any “I” shapes.) Cells with an ‘X’ (if given) are not part of any region.

(Rules and example from PGP IB)

History of the Puzzle[edit]

First appeared on Nikoli volume 104 (2004). Invented by Naoki Inaba (Japan). Originally named ヌルオミノ ("Nuruomino") after ヌル ("to paint") and オミノ ("-omino"). Renamed to "LITS" in volume 112 (2005).



Invented by Robert Vollmert (Germany). First appeared on his blog post in February 2014.[1]

Shade some cells, so that each region contains either no shaded cell at all or exactly four connected cells that form a tetromino. No 2×2 square can be completely shaded or completely unshaded. All tetrominoes must be connected. Tetrominoes with the same shape, regardless of rotation or reflection, cannot share an edge.

(Rules and example from WPC 2019 IB)

Appearances in the past WPCs[edit]