> a chessboard with dominoes when the board is missing two opposite corners. A tricker version asks what square remains (unique up to symmetry) when covering a

> a chessboard with dominoes when the board is missing two opposite corners. A tricker version asks what square remains (unique up to symmetry) when covering a chessboard with 21 trominoes, each of which covers 3 adjacent board squares, i.e. 1x3 or 3x1.