Alternating Inference Chain (AIC)
A short chain of strong and weak links whose matching endpoints eliminate a candidate.
What it is
An Alternating Inference Chain, or AIC, follows candidates through a strict rhythm: strong link, weak link, strong link, and so on. A strong link means one end must be true; a weak link means both ends cannot be true. When the chain starts and ends with the same digit, any outside cell that sees both endpoints cannot contain that digit. The two endpoints cover both possibilities, so the outside candidate is ruled out without guessing.
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.
Worked example
Step through the reasoning below. Each step adds one layer: first the position, then the pattern, then the candidates involved, and finally what can be eliminated.
Step 1 of 4: The starting position
- Pattern cells
- 1/4, 1/7, 2/9, 4/1, 4/9, 8/1, 8/4
- Digits
- 1
- Eliminations
- 4/1
Press a marked square to point it out on the board above.
Why it matters
AIC turns several named expert patterns into one readable language. It is more flexible than a single fish or wing, yet the proof stays local: every link is visible on the board and every elimination has a precise reason. The hint shows solid strong links and dashed weak links so you can trace the argument yourself.
Practise now
Solve a puzzle where this technique decides the game
Frequently asked questions
What is the difference between a strong and weak link?
A strong link says one of its two candidates must be true, such as the only two places for a digit in a unit. A weak link only says the two candidates cannot both be true, such as two equal digits sharing a unit.
Why do the AIC endpoints have to use the same digit?
The endpoint elimination concerns one candidate value. If both endpoints carry that value and an outside cell sees them both, that outside candidate conflicts whichever endpoint the chain forces to be true.
Is an AIC a guess?
No. Every edge is reconstructed from the visible candidates in the current grid. The chain is accepted only when the links alternate correctly and the complete elimination set follows from the endpoints.
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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