Motif Ledger
A motif is a pattern that comes back: a fork, a pin, a piece offered so another can strike. Each one below has a rule written in plain words. The board checked every move of every great game against those rules, so you can look at any tag and see for yourself why it is there.
An offer (4)
Rule: The piece that moved can be taken by something cheaper, or taken with nothing to take back, and it is worth at least three more than anything it captured.
Fork (4)
Rule: The piece that moved now attacks two enemy pieces, each worth more than itself, or the king and a piece worth three or more. Not counted on a mate.
Pin (6)
Rule: After the move, an enemy piece cannot leave its square without exposing its king to a line piece. Counted only when the pin is new.
Discovered attack (3)
Rule: By moving out of the way, a piece lets a bishop, rook or queen behind it attack the king or a piece worth three or more.
Double check (2)
Rule: After the move, two pieces attack the other king at once, so only a king move can answer.
Check (11)
Rule: After the move, the other king is attacked.
Mate (3)
Rule: After the move, the other king is in check and has no legal move.
Mate on the home rank (2)
Rule: A mate where the king is still on its own first rank.
The shelf is small on purpose: three games, each with its moves checked in two sources. The rules are deliberately simple and strict, so some moves a commentator would call a sacrifice (a knight for a pawn, say) do not meet the “offer” rule. That is the honest price of a rule anyone can check.