The third-side rule and safe edges

A safe edge leaves every unclaimed box with at most two sides. An avoidable third side lets the next player close that box.

Rules and strategy methods

If h:0:0 and v:0:0 already exist, adding h:1:0 creates the third side of box (row 1, column 1); another open edge is safer when available.

Guard checks this local condition. A safe edge is not a proof of a globally winning move.

Updated 2026-08-08 · Method source: public-domain Dots and Boxes rules. Approved archive: daily-v2 with exact validation at 8-12 remaining edges; general exact solving is limited to 12 or fewer remaining edges.