Junior Exocet

A Junior Exocet (JE) is one of the most powerful named patterns in classical Sudoku solving. When it fires, it pins two specific digits into two specific cells without telling you which goes where — and that small uncertainty drives a cascade of eliminations across the rest of the band.

JE is the entry point to a small family of related patterns. This page is about the Junior form; see Double Exocet for the coupled variant that's much more common in genuinely hard puzzles.

What's in the family

The Exocet family was developed and catalogued through community discussion starting in the late 2000s. The names below are the conventional solver-community labels — different sources sometimes disagree on the boundary between variants, so treat them as a vocabulary rather than a strict taxonomy.

• Junior Exocet (JE) — base and target cells live in the same band of three boxes. Detected and explained in the app.
• Double Junior Exocet — two JEs paired by their shared base digits. The pairing produces extra eliminations neither JE finds alone. Detected and explained as Double Exocet.
• Senior Exocet (SE) — target cells are allowed anywhere on two cross-lines, not constrained to the base band. Much harder to spot by hand and rarely productive. Not currently implemented.
• Triple Exocet, Compound Exocet, Mutant Exocet, Almost Exocet… Specialised variants that show up in deep solver discussions. Not implemented; included here only so you know they exist if you go reading further.

The Junior Exocet pattern

A JE binds five pieces of geometry:

1. Base cells (B1, B2) — two cells in the same mini-line of one box (e.g. R1C1 and R1C2). Their candidate sets union to exactly three or four digits, called the base digits. The true solution of one base is one of those digits; the true solution of the other is a different one.
2. Target cells (T1, T2) — one cell in each of the other two boxes of the same band, on mini-lines parallel to the base mini-line. Each target must list every base digit as a candidate.
3. Companion cells (C1, C2) — the third cell of each target's mini-line. The companions are the "rest of the line" the targets sit on.
4. S-cells — the six cells outside the base band, in the three cross-lines spanned by the base + targets + companions. These are the cells where the base digits "escape" outside the band.
5. Cover houses — rows or columns. For every base digit, every appearance of that digit in the S-cells must fit inside at most two cover houses. This is the structural invariant that makes JE detection possible at all — without it the search space is enormous.

When all five hold, the targets jointly carry two of the base digits, one each, with the bonded mini-lines and S-cells ruling out everything that would contradict it.

What you can eliminate

The community catalogue lists 11 numbered elimination rules for JE. The app surfaces the four most productive ones — together they account for the vast majority of practical eliminations:

• Rule 3. Every non-base candidate in T1 and T2 is false. This is the primary, always-on elimination.
• Rule 4. Once one target is reduced to a single base digit, the other target cannot hold the same digit (the two targets carry different base digits).
• Rule 10. A base digit that's known to be one of the two true base digits is false in every cell that sees both base cells, or both target cells.
• Rule 11. A base digit is false in every non-S cell of its cover houses. This is often the deepest deduction the pattern delivers, and frequently the move that breaks an otherwise impossible puzzle.

The mirror-node rules (Rules 5–9 in the canonical numbering) catch roughly 10 % more eliminations on top of the four above. They're more delicate to validate and are not yet surfaced in the app.

When you'll see it

Junior Exocets are rare in everyday solving — well under 1 in 1 000 puzzles at typical difficulty levels. They show up almost exclusively in:

• Curated "hardest puzzles" catalogues used by the solver community.
• Championship-grade construction (the World Sudoku Championship and similar events have used Exocet-required puzzles).
• Computer-generated diabolical puzzles that intentionally avoid chain-solvable layouts.

If a puzzle you're playing fires this technique, you're almost certainly working through an explicitly hard-rated puzzle.

How to spot one by hand

1. Look at every box. In each box, look for mini-lines (rows or columns) where the two empty cells share or jointly carry three or four candidates — these are your base-pair candidates.
2. For each candidate base pair, check the other two boxes in the band. Look for target pairs on parallel mini-lines where each cell lists every base digit.
3. Check the S-cells (the six cells in the cross-lines outside the band). For every base digit, count how many rows or columns its appearances span. If any digit hits three or more rows/cols, it's not a JE.
4. If all base digits stay within two cover houses each, you've got a JE. Apply Rule 3 first (strip non-base candidates from the targets); then look for Rule 11 chances (non-S cells in the cover houses).

Related techniques

• Double Exocet — Two JEs sharing base digits. Much more common in hard puzzles than a standalone JE.
• Almost Locked Sets — The other major "structural template" technique. ALS shares Exocet's "two cells, but we don't know which carries what" flavour, on a different layout.
• Death Blossom — Another member of the extreme-difficulty exotic family.

Where the term comes from

The pattern is named after the Exocet anti-ship missile (a French weapon name that became a household word in the early 1980s). The naming reflects the pattern's "lock-on" character: once the structural conditions are met, the targets are pinned with no real freedom left. The rule numbering the app uses in its explanations (Rules 3, 4, 10, 11, etc.) follows the convention established in the long-running community catalogues of the pattern.