Practise: Alternating Inference Chain (AIC)
This puzzle is picked so that the easy moves run out and exactly the Alternating Inference Chain (AIC) gets you going again. Solve as far as you can — when nothing obvious is left, the pattern is waiting for you.
Skip the obvious cells and start right where the technique is needed.
How to spot it
Start with a digit that has a conjugate pair in a row, column or box, or with a bi-value cell. Follow the alternating links without revisiting a candidate. Check the two endpoints: they must share a digit. Finally, look only at cells that see both endpoints; those are the only possible eliminations.
Put it to use
Where you will meet it
Expert grids are the only place this pattern regularly decides the solve. Bring your notes.
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Type the position into the solver and it walks the whole solution, naming the technique behind every step — including this one.
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