Chapter 3 · Mobility

Openness and the Center Split

Openness is the number of empty squares around the discs flipped by this move. An empty square touching several flipped discs is counted only once. The squares around the newly placed disc are not included.

The lower the openness, the fewer new footholds you give the opponent, because empty squares next to flipped discs easily become the opponent's next moves. It is a different criterion from the number of legal moves available right away.

Black F4 flips five discs: C4, D4, E4, E5 and F5. Counting the empty squares next to them, B3, D3, F3, G4, B5, G5 and G6, once each gives seven squares: openness 7. Press ▶ to play it, and the counted squares are shown as blue boxes.

Center split

If every flipped disc is an inner disc whose eight neighbors are all filled, the openness is 0. Such a move is called a center split, the most typical quiet move. It splits a central group of discs and flips only inner discs, so it gives the opponent no new foothold at all.

In the same position, Black D3 flips D4 and D5. Both are surrounded by discs, so the openness is 0: a center split. The engine predicts +2 for D3 and -17 for F4. Compare with the first board.

Exercise

Black to move. Find the move with the lowest openness. If several moves share the same value, all of them are correct.

Exercise

Black to move. Find the move with the lowest openness. If several moves share the same value, all of them are correct.

Exercise

Black to move. Find the move with the lowest openness. If several moves share the same value, all of them are correct.

Exercise

White to move. Find the move with the lowest openness. If several moves share the same value, all of them are correct.

Exercise

White to move. Find the move with the lowest openness. If several moves share the same value, all of them are correct.

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