A 10×10 board by the numbers
A 10×10 Star Battle is defined by 10, 2 and 10: ten rows and columns, two stars in each, ten regions. That is twenty stars on a hundred cells — exactly one in five. The other eighty cells will end up marked X, and most of your solving time goes into placing those X's for the right reasons.
The number that changes the feel of the board is the region count. Ten regions on a ten-line grid means more ways for a region to straddle two or three lines, more places where the row-count and the region-count disagree, and more chances for the arithmetic to hand you a free elimination. Every deduction you know from a 9×9 still works here. What changes is how often the counting ones fire.
The Two Not Touch size
If you first met this puzzle in the newspaper, this is the board you know. The New York Times publishes it as Two Not Touch, on a 10×10 grid, with the same three rules: two stars per row, two per column, two per outlined region, and no two stars touching — not even at a corner. Competition solvers and puzzle-hunt setters call the same game Star Battle. The name changes; the board doesn't.
Two things are different here. The Times prints its board and it is gone with the paper; ours stay playable for good, and the archive holds every 10×10 we have released. And every board here is checked by our solver to have exactly one solution reachable by logic alone, so if you ever find yourself guessing, you have missed a move — and the step-by-step solution will show you which one.
What one more row changes
Going from 9×9 to 10×10 adds nineteen cells, two stars and one region. On paper the board is nearly a quarter bigger. In the hand it feels like more, for three reasons.
Regions overlap more lines. A region averaging ten cells rarely fits inside one row or column, so the simple "confined region" opening that cracks so many 9×9 boards is scarcer. You need the two-line version — a region trapped inside a pair of rows — which is a counting argument rather than a glance.
Bands are wider. Any three adjacent rows hold six stars. On a 10×10 there is enough width for three or four regions to be caught inside such a band, and comparing what they need against what the band holds is the move that opens the middle of the board.
The cascade starts later. On a 9×9 the board tips into forced moves past the halfway mark; on a 10×10 you often place your twelfth or fourteenth star before it does. The extra patience is the whole difference in feel.
The moves that matter at 10×10
All three live in the strategy guide; these are the ones a 10×10 leans on hardest.
Counting a two-row band
Two rows, four stars. If two regions sit entirely inside them, those regions need all four — and every other cell in the two rows is X'd. With ten regions there are more candidate pairs, so check every adjacent pair of rows and columns on a fresh board.
1 The top two rows hold exactly four stars — and these two regions, which fit entirely inside them, also need exactly four.
2 Those regions consume the whole quota, so every other cell in the two rows is X'd.
Virtual bands
Widen the idea: three rows hold six stars. Total up what the regions caught fully inside those rows must contribute, subtract, and you know exactly how many stars are left for the cells that poke in from outside. This is the workhorse of the 10×10 middle game and the move that makes the bigger board feel different from the smaller one.
The bottom three rows form a band holding exactly six stars. The two highlighted regions fit fully inside it and claim four — leaving just two stars for every other cell in the band, an accounting no single row reveals.
The star claim
Once the X's squeeze a region's remaining cells into a single row or column, that region claims the line's two stars — even before you know which cells they are — and the line is cleared everywhere outside the region. On a 10×10 this often fires two or three times in a row.
1 Earlier X's have squeezed this region's open cells into a single column — both of its stars must land there.
2 The region claims the column's two stars, so the column is X'd everywhere outside it.
A typical 10×10 solve
The same three phases as a 9×9, with a longer middle.
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1
The opening: shapes and pairs
Scan the ten regions for anything small or thin — a sliver forces both its stars at once, a compact block gives up its centre. Then look for a region trapped inside two rows or two columns. On a 10×10 the opening is shorter than you'd like; six to ten marks is normal.
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2
The middle: count the bands
This is the phase that makes 10×10 its own game. Work through the two- and three-row bands, tally the regions inside each, and X out whatever the arithmetic excludes. Then sweep for completions. Expect to loop between the two several times.
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3
The finish: the cascade
Around the fourteenth star the board tips and the rest falls in a rush. If it doesn't tip — if you have fifteen stars and an open, silent board — go back to the bands. On a 10×10 the missed move is almost always a count, not an X.
Coming from the newspaper?
A few things work differently on a screen, and all of them in your favour. Tap a cell once for an X and again for a star, so there is no pencil smudge to read through. With Auto-mark Xs on (the default), the board marks every cell your stars rule out the instant you place them — the no-touch ring, the completed rows — which lets you spend your attention on the counting. The board checks itself when the last star lands, and yesterday's puzzle is still here if you want to finish it.
Prefer paper after all? The printable books put four boards on a page, ready for the kitchen table.
10×10 or 9×9?
Choose 10×10 for the board you know from the paper and for a longer, counting-heavy solve. Choose 9×9 for a daily habit and for drilling technique, where every move is quick to see in hindsight.