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Explanation
The □ sign is read as “only move.” It warns that the reasonable alternatives fail, allow a serious loss, or do not meet the position's specific demand. The marked move may be defensive, tactical, or apparently quiet.
In , the text format for games, the original specification distinguishes Numeric Annotation Glyph (NAG) $7, a forced move because all others lose quickly, from $8, a singular move with no reasonable alternatives. Scid vs. PC displays □ for $8, while a printed source may use the square more broadly. Reading the underlying number preserves the distinction.
“Only” does not always mean the only legal move. A king may have several rule-compliant replies, yet only one avoids mate or maintains the balance. The symbol also records the judgment of the available analysis; deeper examination may uncover another defense.
Common confusions
Only legal move
Several moves may be legal under the rules while only one avoids a decisive deterioration.
Excellent move
□ describes necessity, not beauty. An obvious or unpleasant move can be the only one that works.
Sources
- 1.System of Signs, Chess Informant / ChessBase
- 2.Standard: Portable Game Notation Specification and Implementation Guide, Steven J. Edwards (coordinator)
- 3.NAG Values, Scid vs. PC