The set consists of equal three-cell corners ( $L$ -triminoes), the middle cells of which are marked with paint. A rectangular board has been covered with these triminoes in a single layer so that all triminoes were entirely on the board. Then the triminoes were removed leaving the paint marks where the marked cells were. Is it always possible to know the location of the triminoes on the board using only those paint marks? Alexandr Gribalko