Map
Can regions touching at a corner share a colour in Map?
Yes. The four-colour rule separates regions sharing a length of border. Point contact alone creates no colour restriction. Keep the given region colours unchanged.
Worked example
Two red regions meeting only diagonally are legal. If they share even a short edge, one must have another colour unless both are fixed clues, indicating another deduction was wrong.
What to check on your board
Apply the rule above to your current board, then verify the remaining puzzle constraints. The example explains a local deduction; it is not a solution to every randomly generated board.
Checking your next move
Follow the actual shared border between two regions before excluding a colour. A corner touch is easy to mistake for a short edge. Keep fixed colours as reference points and revisit an earlier deduction if two fixed regions appear to require an impossible change.
Using this reference
For Map, read the rule and the worked example together before opening the game. Separate what the rule explicitly requires from what only seems likely on the current board. A useful next move is one you can explain from the visible constraints, not merely a pattern that looks familiar.
If you get stuck, isolate a smaller part of the position and list what is still possible there. After a move, check the affected rows, regions or connections again. The reference explains the documented rules; progress, controls and save behavior belong to the linked game.
Read the developer’s manual ↗ · Open the developer’s browser version ↗
Manual inspected: · Simon Tatham & contributors · MIT