only possibility for that specific number within the line or block, so it has to be that number to satisfy the "have numbers 1-9 in every column, row, and block".
naked doubles are a pair of cells in a block, column, or row that have the same 2 numbers and no others after pencil marks. all other cells in the same block/column/row as the naked double can have the numbers of the double erased from their candidate list.
in the example above the 4 and 6 make a naked double forcing the other cells in the top right block to lose their 4 and 6. also because of the placement, if there 