Palindrome Sudoku Lines Explained in One Short Example
A gray line runs diagonally across three cells. It starts with 7 and has a 2 in the middle, so the far end must be 7 as well.
Transcript
The rule
A palindrome line is a gray line that reads the same forward and backward. The first and last cells match, the second and second-to-last match, and so on toward the middle.
Each mirrored pair is one digit written twice. Anything you learn about one cell of the pair is true of the other, so the line carries information across the grid.
Worth memorizing
Cells in mirrored positions hold the same digit.
So mirrored cells can never share a row, column or box.
An odd-length line has a free middle cell.
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