On: The rarest move in chess [video]

[yunwal]

Zugzwang has no notation. The author states in the first 2 minutes or so that the video is about the rarest edge cases in notation.

[CalChris → yunwal]

Nf3 can be ambiguous but resolved by the position itself. That ambiguity has no notation either.

FWIW, I wrote the XBoard PGN parser.

[yunwal → CalChris]

Right, the video talks about that and doesn’t consider the ambiguous but resolved by position one different from any other Nf3

[mathgradthrow → yunwal]

parent is referring to stalemate. Underpromotion to a bishop to achieve stalemate was my guess for rarest, since you can always stalemate with a queen just as well as a bishop, so theres no reason to underpromote if you goal is a draw on the turn in which the promotion occurs.

[yunwal → mathgradthrow]

Stalemate doesn’t have notation either, so the OP doesn’t consider it different than any old bishop underpromotion