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Sudoku technique · Short chains · first needed in Tricky puzzles

2-String Kite

A two-location row and a two-location column connect through two distinct cells in one box. At least one remote endpoint must be the digit.

How to spot it

Look for one row string, one column string, and a shared-box hinge.

The common mistake

The target sees both remote endpoints, not a hinge cell and an endpoint. The two hinge cells must be distinct.

Learn this first

Strong and weak links.

A worked example

This position comes from SudokuByte’s lesson on the 2-String Kite. Gold cells hold the evidence; green cells are the pattern’s ends.

The 2-String Kite in this position
The result: pencil marks it removes, in red

  1. First, follow the 1s in row 7

    Look at the whole row 7, including its filled numbers. Its only 1 pencil marks are r7c1 and r7c3. One of those two must be 1. We do not know which.

  2. Find the second two-location line

    column 2 has only two 1s: r8c2 and r6c2. One of these must be 1. These two-location constraints are called strong links.

  3. The two lines meet through a box

    r7c3 and r8c2 share box 7. They cannot both be 1. This mutual exclusion connects the two strings. The remote ends are r7c1 and r6c2.

  4. Try both cases, without guessing

    Suppose r7c1 is not 1. Its line forces r7c3 to be 1; the box rules out 1 in r8c2; the other line then forces r6c2 to be 1. So either r7c1 is 1, or r6c2 is. At least one far end is 1.

  5. Make one deduction, not a guess

    Erase 1 from r6c1. This does not tell you which far endpoint is 1; it only proves the target cannot be. Apply this change when ready.
Practice the 2-String Kite on a real boardPlay a Tricky puzzle

More short chains

Further reading: hodoku.sourceforge.net