The safe phase
At the start, any line that does not draw the third side of a box is safe. Both players draw safe lines until there are none left. At that moment the board has split into chains, rows of boxes in which each box has two sides drawn, and possibly loops, chains that close on themselves.
Who opens the first long chain
Once safe lines run out, the player to move must give something away. If short chains of one or two boxes remain, he gives those first. When only chains of three or more boxes are left, the player who must open one loses most of it, because the opponent takes the boxes and then decides what happens next.
So the real fight in the middle game is about parity: making sure that when the safe lines run out, it is your opponent who has to move.
Declining: the key technique
Suppose your opponent opens a long chain. You take all its boxes except the last two, and then, instead of taking them, you draw the line at the far end of the chain. That hands two boxes to your opponent, who takes them and must now open the next chain for you. You gave up two boxes and kept control of everything else.
With a loop the same idea costs four boxes, because a loop ends in two pairs. That is why loops help the side without control.
Giving boxes the right way
- When you must give away a single box or a pair, draw the line that leaves no way to decline. For two boxes, draw the middle line between them so your opponent cannot hand them back.
- Give the shortest chain available.
- Sacrificing a box earlier can change parity and make your opponent the one who opens the long chains later.
A simple habit
From the moment the board starts to fill, count the chains of three or more boxes after each move. When you know how many there will be, you know whether you want to keep the number as it is or change it with a sacrifice. Books on the game call this counting the long chain rule; the habit matters more than the formula.
Practise it
Play the 5 × 5 board against the Knight, then work through the hard dots and boxes problems, many of which are solved by declining at the right moment.