draw by perpetual check

Technically, there is no "draw by perpetual check" rule in chess.

Practically, if the check really is perpetual, then eventually there will be a three-fold repetition of the position or 50 irreversible moves, and you can claim a draw.

threefold repetition means the POSITION has been repeated (exactly) three times.  If you look, there 'offer draw' button will have changed to a 'claim draw' button.

Shouldn't that be 50 reversible moves, because in most cases of threefold repetition, the moves essentially "undo" themselves? Pawn moves cannot give rise to position repetition, so they are truly irreversible.

@KyleMayhugh: Boo. I meant to say 50-move rule, not threefold repetition. Sorry.