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Level 3 sudoku puzzles on this website can be solved using advanced techniques, such as X-wing, XY-wing, XYZ-wing, skyscraper, Kite, crane, swordfish, and jellyfish, plus level 1 and 2 techniques.
This page is especially for training with crane pattern only. For training with other patterns, check the links below.
Principles of the crane pattern:
Suppose a number can only appear in two squares {A, B} in a row (column). Also suppose this number can only appear in two squares {C, D} in a block. If {B, C} are on a same column (row), then this number cannot be in the squre that is on the same column (row) with {A} and on the same row (column) with {D}.
For the above example of crane pattern, the number {3} can only appear in the red squares in the 9th row. And this number can only appear in the two red squares in the 1st block. Two left side red squares are on the same column. So this number {3} cannot be in the yellow square which is on the same column with the red square R9C8 and on the same row with the red square R1C3.
Why? Because either the red square R9C8 or the red square R1C3 must have the number {3} in it!
In the followig example of crane pattern, the number {4} cannot be in the yellow square.
