Block and Column / Row

Sometimes, when you examine a block, you can determine that a certain number must be in a specific row or column, even though you cannot determine exactly which cell in that row or column. This is enough information to remove that number from the candidate list for other cells in the same row or column, but outside the block.

For example, in the partial Sudoku puzzle below, the 7 in the top-left block can only occur in column two. We can't say whether it's in r1c2 or r3c2, but it must be in one of them. This means 7 cannot occur in any other cells in column two, and so we can eliminate 7 from the candidates for the marked cells.