How One-Per-Region Logic Puzzles Work

One-per-region puzzles combine a regular square grid with irregular colored regions. Your task is to place markers so several simple quotas are satisfied at the same time.

The rules are compact. The interesting part is how they overlap.

The four core rules

In a typical single-placement board, place exactly one marker in each:

• row;
• column;
• colored region.

Markers also cannot touch one another, including diagonally.

In Nimblins, the markers are collectible characters. Easy and Medium Classic puzzles use the single-placement rule. Hard and Expert raise the quota to two Nimblins in every row, column, and region while keeping the no-touch rule.

Why regions change the puzzle

If the board only required one marker per row and column, many arrangements could work. Irregular regions add a third set of constraints.

Every cell belongs to three units at once:

1. its horizontal row;
2. its vertical column;
3. its colored region.

A placement that satisfies one unit can restrict the other two. That interaction produces the puzzle’s chain of deductions.

Imagine a three-cell region shaped like an L. One cell sits in a row that is already complete. A second touches an existing marker diagonally. The third cell must contain the region’s marker. You did not need to inspect the full board—two local exclusions solved the region.

The no-touch rule does more than prevent clusters

When markers cannot touch, a confirmed placement immediately excludes up to eight neighbors: above, below, left, right, and the four diagonals.

This rule creates especially useful patterns.

Adjacent cells in one row

If a region’s only two candidates are side by side in the same row, the exact position may be unknown. But either choice blocks nearby cells in the rows above and below. You can sometimes mark cells that touch both candidates even before choosing between them.

Candidates inside a narrow region

If every candidate for a region occupies two neighboring columns, those columns may be strongly constrained. The region must use one of them, so other units cannot treat both columns as completely free.

The point is to use what all remaining candidates share, not only what differs between them.

The first deductions to look for

A completed row, column, or region

Once a unit has its marker, every other cell in that unit becomes an X. This is the fastest source of new information.

A unit with one legal cell

Count the open candidates, not the empty-looking cells. A cell ruled out by spacing, a completed crossing unit, or another region is no longer a candidate. If one remains, place the marker.

A region confined to one row

Suppose a region still needs one marker and all of its candidates lie in row 4. Row 4’s marker must come from that region. Other cells in row 4, outside the region, can be excluded.

The same reasoning works with a region confined to one column.

A row confined to one region

Turn the previous idea around. If every candidate for a row lies inside one region, then that region’s marker must occupy one of those row cells. Other cells in the same region can be excluded.

A small worked example

Consider row 3 on a 5×5 board. It needs one marker. The five cells have these states:

1. column 1 is already complete;
2. column 2 remains open;
3. column 3 touches a marker diagonally;
4. column 4 belongs to a completed region;
5. column 5 touches a marker horizontally.

Only column 2 is legal, so place the marker at row 3, column 2.

Now apply consequences immediately:

• mark every other cell in row 3;
• mark every other cell in column 2;
• mark every other cell in that colored region;
• mark all neighboring cells around the new placement.

The placement may have solved one cell, but its consequences can create several new singles.

How two-per-region puzzles differ

Double-placement rules do not change the basic vocabulary. They change the counting.

A row with one Nimblin is not complete; it still needs one more. A region needing two markers can force both remaining candidates. Spacing also means the two placements in a unit cannot occupy touching cells.

The safest habit is to state the remaining quota rather than asking whether a unit “has a marker.” For example:

• row 6 has one of two, so it needs one;
• region blue has zero of two, so it needs two;
• column 3 has two of two, so every other cell is closed.

This keeps single-rule habits from causing mistakes on harder boards.

A reliable scan order

When a board stalls, use this order:

1. completed regions;
2. completed rows;
3. completed columns;
4. spacing around every marker;
5. regions with few candidates;
6. rows and columns with few candidates;
7. region–row and region–column interactions.

Record X marks as you go. The board becomes easier to read because the candidate set becomes explicit.

For more detail, continue with practical solving strategies and common mistakes that interrupt deduction chains. You can also explore Nimblins to see how these rules expand across its modes.