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Let's Make Number Place (Elementary course)
Strategy Guide
Number Place Strategy Guide
X-Wing
Pay attention to the locations where two vertical and horizontal lines meet. On some occasions the same number can be entered twice in the four squares at the intersections of the lines. Since a number cannot be entered twice on either the same horizontal or vertical line, the number must be entered diagonally from itself in either the upper left and bottom right intersection squares or upper right and bottom left intersection squares. Furthermore, that number cannot appear anywhere else in either of the two vertical or horizontal lines.
You can use this method to remove candidates from a number of squares.
Example 1
Pay attention to the number 1 and the two yellow, horizontal lines.
Due to the effect of the previously entered numbers, the only locations where you can enter the number 1 in the two yellow lines are in the blue squares.
1 can only be entered in one of two patterns: as indicated below by either two blue numbers or two violet numbers.
Though there are two ways to enter these numbers, they both form on opposing lines. If you were to draw a line between each of the possibilities, you'd see an "X," which is the root of the name "X-Wing."
If you now look at the vertical lines, you'll see that the number 1 can also only be entered in one of the two blue squares in each vertical line. Therefore, 1 cannot be entered in any of the other squares in the vertical line.
We've indicated the possible candidates for each square below. View the X-Wing for yourself.
The number 1 will be entered on the two diagonally opposing squares of the four possible candidate squares. Therefore, none of the other non-blue squares in the same vertical line as the blue squares can be 1. You may therefore delete 1 from the candidates of the other squares in the vertical line.
In the diagram above you can see that 1 was deleted from three squares in each of the columns.
Since this technique only removes possible candidates you may think that it may not help you much in your games. However, by removing candidates you will be able to place numbers more quickly.
Though this is quite a high level technique, you will need to know it to solve more difficult puzzles.