Every option agrees

Page 4 of How to Solve Star Battle Puzzles.

A small region has only a few places for its star. Every option agrees tries each of them. Whatever every option does, you can do now, without knowing which option is right.

Two things can happen:

A square that every option rules out gets a dot.
A square that every option uses must be a star.

In one-star puzzles, the app's hints only ever use the first. The second shows up from two stars up.

In the app's hints, 39% of 8×8 and 49% of 10×10 one-star puzzles use this move for regions with two or three options. The bigger case below is in 24% of 8×8 and 34% of 10×10 puzzles.

A region with two or three options

Look at this region of three squares. Its star goes in one of them.

An 8×8 one-star puzzle.

Every option agrees: Every way that the stars of this region fit rules out this square. Rule it out.

The region of three squares fits its star in three ways. Every way touches the square at row 4, column 3, so that square gets a dot. The new dot is in blue.

Try each option in turn:

A star at row 4, column 2 touches row 4, column 3.
A star at row 5, column 2 touches row 4, column 3, at a corner.
A star at row 5, column 3 touches row 4, column 3.

Every option touches that square. Stars cannot touch, so no star can go there. This works because the three squares and the square that gets the dot make a 2×2 block, and a 2×2 block holds at most one star.

A bigger region: test one square

Some regions have four, five or more options. You cannot try them all. Test one square instead. Ask what a star there would do to the region.

Every option agrees: A star here touches every way that this region can fit its stars. Rule it out.

This region has four squares. A star at row 2, column 2 would touch all four, so that square gets a dot. The new dot is in blue.

If a star in one square touches every square of the region, the region would be left with no place for its star. So the square cannot hold a star. The hint puts it this way: "A star here touches every way that this region can fit its stars."

The hint shows the square to test. You can find it yourself. Look at a square that touches many squares of a small region, and check whether it touches all of them.

What to look for

Small regions. Start with a region of two to five squares.
A square next to the region. The square that you rule out is often just outside the region, in a corner of it.
Then check every square of the region. If one is out of reach of the test square, the test fails. That is fine. Try another square.

Practice on puzzles like these in our Star Battle Puzzles app, for iPhone, iPad, Mac and Android.

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