# Hidden Quad

A Hidden Quad occurs when four digits are candidates only in four cells of a house. Other candidating digits can be eliminated from these cells.In the sudoku diagram below, the digits 3, 4, 7 and 8 in the upper right box can only be placed in the R2C7, R2C9, R3C7 and R3C9 cells. The remaining digits 1, 2 and 5 can be eliminated from these cells. Thanks to this elimination, the digit 2 in the second row can only be placed in the R2C1 cell.