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November 7, 2014
Peers    find this word on this page for a good Sudoku Puzzle tip summarized here and more
 There are 9 cells in a house. Every cell is related to the 8 other cells in that house. A cell is a member of 3 houses: a row, a column and a box. 2 cells in the box are also a member of the same row, and 2 cells are also a member of the same column, leaving 20 cells in total that share one or more houses with any other cell.
We call these 20 cells the “peers” or “buddies” of that cell. You could also say that the cell can “see” those 20 cells
When a cell,R5C5, contains a digit, it has an effect on all peers. None of those 20 cells can contain the digit that is placed in R5C5. This is what we call forward reasoning. It starts from a placed digit and looks at the effects that the placement has on the peers.
 However, when the cell is empty, we can reverse the logic. We start with the empty cell and scan the peers to determine what digits it cannot contain. Whatever remains can be placed in the cell. You will have to learn looking at the grid in both directions. It is the most important tool for a Sudoku solver. You will be able to practice this again and again, so don’t worry if you are not really familiar with it yet.When a cell contains a digit, there is yet another conclusion to be drawn, so obvious, that many people overlook it. The cell can no longer contain any of the other 8 digits. This conclusion is useless for the filled cell itself, but very useful for each of the houses the cell belongs to.

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