Region inside a line and Line inside a region

Page 2 of How to Solve Star Battle Puzzles.

When Star neighbors, Finished and Last squares stop, look at where the open squares of each region sit. Two moves come from that. They are mirror images of each other.

This page teaches the easy form of both, where all the open squares are in one line. A line is a row or a column. Harder forms, where the squares are in two or more lines, come later in the guide.

You'll meet Region inside a line in about 9 of every 10 one-star puzzles. You'll meet Line inside a region in about half of them. (In the app's hints, 92% of 8×8 and 94% of 10×10 one-star puzzles use the first, and 54% and 49% use the second.)

Region inside a line

Suppose all the open squares of one region are in one row. The region needs a star, and every place it can put one is in that row. So its star goes in that row. A one-star row holds only one star, so the rest of the row cannot hold one. Rule out the rest of the row.

An 8×8 one-star puzzle.

Region inside a line: All the open squares of one region are in this row. So the stars left in this row must all be in that region. Rule out the rest of this row.

The region of three squares in row 5 has no square outside that row. Its star is in row 5, so the rest of row 5 gets dots. The new dots are in blue.

It works the same on a column. When the open squares of a region are all in one column, rule out the rest of the column.

Line inside a region

Now turn it around. Suppose all the open squares of one row are in the same region. The row needs a star, and every place it can put one is in that region. So that region's star is in this row. The region cannot hold another, so rule out the rest of the region.

Line inside a region: All the open squares of one row are in this region. So this region holds all the stars of that row. Rule out the rest of this region.

Every open square of row 1 is in the large region along the top. That region gets its star from row 1, so the rest of the region gets dots. The new dots are in blue.

The same works for a column.

Line inside a region: All the open squares of one column are in this region. So this region holds all the stars of that column. Rule out the rest of this region.

All of column 1 is in one region. That region gets its star from column 1, so the rest of the region gets dots. The new dots are in blue.

What to look for

Short lines. After a few dots, a row or column often has only two or three open squares. Check which regions they belong to.
Small regions. A region of two or three squares is usually in one row or one column.
Both directions. Check each region for a line, and each line for a region. A square that you rule out with one move often sets up the other.

When the open squares of a region are in two rows, nothing is ruled out yet. The next move, Regions inside lines, counts several regions and lines together.

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

Next: Regions inside lines
Previous: Star neighbors, Finished and Last squares
All parts of the guide