Star Battle Regions: How Small Regions Solve the Puzzle
How a region's shape decides where its Star Battle star can go. Dominoes, L-shapes, regions inside one row and the N-in-N count, with a 6x6 solve.
Small regions solve Star Battle because each region must hold exactly one star, and a small region has very few places to put it. A region's shape tells you more than its size: a two-square domino owns the star of its row and empties four squares beside it, an L-shape empties the square that would complete it, and two regions packed into two rows empty everything else in those rows. This post walks through each shape, then solves a full 6x6 puzzle with nothing but region logic.
It is written for players who know the rules and have solved a few easy grids. If you are new, start with how to play Star Battle and come back.
Why regions are the strongest clue in Star Battle
In one-star Star Battle you place one star in every row, every column and every region, and no two stars may touch, not even at a corner. That is the version Puzzle Picnic's Stars uses, and the one LinkedIn's Queens uses with crowns instead of stars. Rows and columns are always the same size: on a 6x6 grid each has six squares. Regions are not. One region might cover twelve squares and another only two, yet both get exactly one star.
That makes regions the uneven part of the puzzle, and the uneven part is where the information is. A twelve-square region could take its star in many places. A two-square region has two choices, and every choice it makes has consequences for the row, the column and the neighbours. The habit to build is simple: before you look at anything else, find the smallest regions and ask what their shape forces.
This post names squares by row and column, counting from the top left: r2c3 is row 2, column 3. In the diagrams, letters are regions, * is a star, and x is a square marked empty (in the app, you tap a square once to mark it empty and twice for a star).
What each small region shape forces
Every small shape forces something, and the useful question is always the same: which squares outside the region touch every square inside it? A star in such a square would rule out the whole region, so it can never hold a star. This is the "neighbourhood" idea that runs through all of Star Battle region logic.
| Region shape | Where its star can be | What it rules out |
|---|---|---|
| One square | That square | Its row, column and all eight neighbours |
| Domino across a row | One of two squares | The rest of the row, plus the two squares above and two below it |
| Domino down a column | One of two squares | The rest of the column, plus the two squares on each side |
| Straight line of three | One of three squares | The rest of the line, plus the square on each side of the middle |
| L-shape of three | One of three squares | The fourth square of its 2x2 block |
| 2x2 block | Any of four squares | Nothing outside on its own, but it can only ever hold one star |
| Straight line of four or more | Several squares | The rest of the line only |
The domino is the shape you will meet most on Easy and Medium grids, and it is worth learning by heart: one domino empties its whole row (or column) and four more squares. On a 6x6 grid that is eight of the 36 squares crossed out at one look.
The L-shape matters more than it looks, because regions shrink as you solve. A big region with most of its squares crossed out often leaves an L or a domino behind, and the same rule then applies to what is left. Read the open squares of a region, not its printed outline.
Regions that fit inside one row or column
A region whose open squares all lie in one row owns that row's star. The row needs exactly one star, the region needs exactly one star, and the region can only put it in this row, so the row's star must be the region's. Every square of the row outside the region is empty.
This holds for any size of region, not only dominoes. A long thin region running along the bottom edge does the same job. So does a big region that has been worn down until its last open squares sit in one row. That last case is the one beginners miss: the region looked large at the start, so they stop checking it.
The same works for columns. A region that is three squares tall and one square wide owns its column's star, and every other square in that column is empty, however inviting it looks.
Rows that fit inside one region
The rule also runs the other way round. If every open square of a row belongs to one region, that row's star is that region's star, so the rest of the region is empty.
This is easy to overlook because you are used to regions sitting inside rows, not rows sitting inside regions. It typically appears late in a solve: a row has lost most of its squares to crossed-out neighbours, and the two or three squares that remain all have the same colour. At that point the region's other squares, which might be spread over three rows, can all be marked empty.
Two regions in two rows: counting the stars
When two regions fit entirely inside the same two rows, those two rows' stars are taken. Two rows hold exactly two stars between them. The two regions need two stars, and they can only put them in these rows. So both stars in the two rows belong to those regions, and every other square in the two rows is empty.
The rule grows with the numbers: three regions inside three rows, four inside four. It works for columns too, and it works backwards: if two rows contain open squares from only two regions, those regions' stars are in those rows, and their squares elsewhere are empty. Krazydad's advanced tutorial for Two Not Touch (two-star Star Battle) calls that backwards version the "Rule of Container Cabals". This post calls both directions N in N.
Two things make N in N practical:
- Look for it after the easy marks, not before. It needs squares to have been crossed out first, because a region only "fits" inside two rows once its other squares are gone.
- Start the search from pairs of small regions that sit next to each other. Two dominoes in the same two columns, or an L above a domino, are the usual suspects.
On larger grids this counting is the main tool. Puzzle Picnic's Expert Stars puzzles (9x9) need this kind of reasoning about groups of regions, and Hard ones (8x8) may. Easy and Medium never need a group: one region, row or column at a time is enough.
Squeezes: a star that would starve a region
A square outside a region can be ruled out if a star there would cover every open square the region has left. The domino and L-shape rules above are this idea applied to a fixed shape. The general version works on any region, row or column with only a few open squares.
To check a candidate square, picture a star on it and cross out what that star would rule out: its row, its column, its region and its eight neighbours. If some region, row or column ends up with no open square, the candidate is impossible. With practice you stop picturing it and see it: two open squares side by side in a row are both touched by the two squares above them and the two below.
A squeeze is still pure logic. You ask one yes-or-no question about one square, and the rules give a definite answer; you never place the star and play on to see what happens.
A 6x6 puzzle solved with region logic
The puzzle below has six regions, A to F, on a 6x6 grid (the size of an Easy Stars puzzle). It has one solution, and every step uses one of the ideas above.
Step 1: the two dominoes claim their rows. Region A is r1c3 and r1c4, all in row 1, so the other four squares of row 1 are empty. Region E is r5c3 and r5c4 in row 5, so the other four squares of row 5 are empty.
Step 2: the squares beside each domino. r2c3 and r2c4 touch both A squares, so they are empty. r4c3, r4c4, r6c3 and r6c4 touch both E squares, so they are empty.
Step 3: two dominoes in two columns. A and E both sit in columns 3 and 4. Two regions in two columns means those columns' stars both belong to A and E, so every other square in columns 3 and 4 is empty. That adds r3c3 and r3c4.
Step 4: two L-shapes appear. Region B has only r2c5, r2c6 and r3c6 left, an L. The square that completes its 2x2 block, r3c5, touches all three, so it is empty. Region C has r2c1, r2c2 and r3c1 left, another L, so r3c2 is empty.
Step 5: two regions in two columns again. B's open squares (r2c5, r2c6, r3c6) and D's open squares (r4c5, r4c6, r6c6) all lie in columns 5 and 6. Those two columns' stars belong to B and D, so r6c5, a square of region F, is empty.
Step 6: a squeeze on region D. D's open squares are r4c5, r4c6 and r6c6. A star on r3c6 would touch r4c5 and r4c6 and share column 6 with r6c6, leaving D with nowhere to go. So r3c6 is empty. That leaves row 3 with one open square, r3c1, so the star goes there. It rules out r2c1, r2c2, r4c1, r4c2 and r6c1.
Step 7: the bottom of the grid falls. Column 2 now has one open square, r6c2, so it takes the star. That star empties the rest of row 6, including r6c6, and it touches r5c3, so region E's star must be r5c4. Column 4 is now taken, so region A's star is r1c3. The star on r5c4 also touches r4c5.
Step 8: the last two. Region D has only r4c6 left, so the star goes there, which empties r2c6. Region B's last square is r2c5.
Notice what never happened: no step needed a guess, and the first five steps placed no star at all. Region logic often works like that. You spend the opening crossing out squares, and then the stars arrive in a rush.
A scanning routine for any grid
A fixed order of checks keeps you from staring at the grid. This is a routine that works from 5x5 up to 9x9:
- Mark every one-square region's star and cross out around it.
- Find every region whose open squares lie in one row or one column, and empty the rest of that line.
- For each region of two or three open squares, cross out the squares that touch all of them.
- Look for rows or columns whose open squares all belong to one region.
- Try pairs of small regions for N in N, then triples.
- Only then test single squares for squeezes.
After every star, go back to step 1, because each star removes up to eight neighbours and can turn a big region into a small one. The Star Battle strategies guide covers the other side of the puzzle, counting rows and columns, in the same order.
Frequently asked questions
Which region should I start with in Star Battle?
Start with the region that has the fewest open squares. A one-square region is solved on sight, and a domino or a straight line of three settles its whole row or column. Large regions rarely give anything away until the small ones have been placed.
Can a 2x2 block hold two stars?
No. Any two squares in a 2x2 block touch, either along a side or at a corner, so a 2x2 block can hold at most one star. That is true whether the block is one region or spread across several, which makes it useful for counting on larger grids.
What does "N in N" mean in Star Battle?
It means N regions whose open squares fit inside N rows (or N columns). Those rows need N stars and the regions supply all of them, so every other square in those rows is empty. Two dominoes in the same two columns is the simplest case.
Does region logic work for two-star Star Battle?
Yes, with the numbers changed. In a two-star puzzle every region holds two stars, so a region that fits inside one row owns both of that row's stars, and N regions inside N rows become N regions holding 2N stars. The shape rules change more: a domino can hold only one star, so a two-star region must be bigger.
Where to go next
Region logic is the half of Star Battle that is easy to see once you know where to look. Practise it on small grids, where you can check every step, then move up a size when the routine above feels automatic. If you keep getting a red star, the list of common Star Battle mistakes shows the usual causes. If you came here from LinkedIn's Queens, the same rules apply there, and the Queens and Star Battle comparison shows how the techniques carry over. You can practise on the Stars puzzles in Puzzle Picnic, where a hint explains the next logical step when you are stuck.