Nunn Convention for Endgame Annotations

From the Wikipedia page that explains the Nunn Convention:

In his 1992 book Secrets of Rook Endings and other books in the series (Secrets of Minor-Piece Endings and Secrets of Pawnless Endings), John Nunn uses these symbols in a more specific way in the context of endgames where the optimal line of play can be determined with certainty:

• ! – the only move which maintains the current evaluation of the position: if the position is theoretically drawn, this is the only move which does not lose; if the position is theoretically won, this is the only move which secures the win. An "!" is used no matter how trivial the move in question; the only exception is if it is the only legal move.
• !! – a particularly difficult-to-find "!" move
• ? – a move which negatively affects the evaluation of the position: if the position had been drawn before the move, it is now lost; if won before the move, it is now drawn or lost
• ?? – an obviously bad "?" move
• !? – a move which makes the opponent's task harder or one's own task easier; for example, in a theoretically lost position, a move which forces the opponent to find several "!" moves in order to win
• ?! – a move which makes the opponent's task easier or one's own task harder; for example, in a theoretically won position, a move which requires several subsequent "!" moves in order to win (Nunn 1999)

This convention has been used in some later works, such as Fundamental Chess Endings and Secrets of Pawn Endings by Karsten Müller and Frank Lamprecht, but it can be safely assumed the convention is not being used unless there is a specific note otherwise. The Nunn convention cannot be used to annotate full games because the exact evaluation of a position is generally impractical to compute.
In 1959, Euwe and Hooper made the same use of the question mark, "... a decisive error...".^[3]