Obstacles: holes that make new edges
A sunken cell is a hole in the cloth. Leave it empty — threads can’t go through it. The rest of the cloth still has to be covered completely.
A hole is simple to understand and easy to underestimate. It doesn’t just remove one cell: it changes every cell around it.
A hole is a piece of edge
For its neighbours, a hole works exactly like the border of the board. A cell next to a hole has one exit fewer. So all the edge tricks move inside the board: a cell between a hole and the border is squeezed from two sides, and a cell touching two holes may become a corner in the middle of the cloth.
Count exits as you would on the edge. Two exits on opposite sides — a corridor, the thread passes straight. Two exits on adjacent sides — a corner. One exit — the cell must hold a button.
Holes split the board
A line of holes works like a wall. Several holes in a row, or holes meeting the border, cut the cloth into parts connected only through narrow gaps. Each gap is a strong clue: only a limited number of threads can pass through it, and a part that contains just one button of a pair must be left by that pair’s thread through one of the gaps.
Before drawing, outline the regions in your head. For each region, list the buttons inside. A region with an odd structure — one button and lots of empty cells — tells you which thread has to come in and fill it. The same reasoning solves big boards (/guides/big-boards/).
Don’t forget the rest of the rules
- The level is solved when every cell except the holes is covered. A gap left next to a hole counts as empty cloth.
- Holes don’t belong to any thread; you can’t “park” a thread in one.
- Corners of the board still work as usual — often the first forced move is in a corner, and the holes give the second.
A useful check after every thread: walk around each hole and look at its neighbours. If one of them now has a single free exit and no button, the last thread was wrong. Holes make such dead ends appear faster than on a plain board.
A hole in a corner of the board is a cut-off corner: the two cells next to it become the real corners.
Practise in the Obstacles set: eight levels from 5×5 to 7×7, from boards with a single cluster of holes to boards where holes line up into a wall.