A challenge

It is common in chess for a player to have only one legal move.

Occasionally, as in the position below, there will be a forced sequence of two half-moves, i.e. only one legal move and only one legal reply.



Black must play Rxh7+, white must reply Bxh7. Neither has any other choice (except resignation etc).

What is the longest forced sequence you can create? N.B. moves must be only legal moves, for both sides.

You're going to get some pretty horrible positions. Best I could get, in all of 1 minute thinking, was this:
15 forced half moves.

3 half moves. (Black first)

Originally posted by DeepRed
[fen]K7/3p4/2pP4/k1P5/Pp6/1P6/8/8[/fen]
3 half moves. (Black first)

Originally posted by d36366
Are you sure? Black's 1st move is forced - Ka6 but then white has a choice of either Kb8 or a5, doesn't he?

Originally posted by doodinthemood
You're going to get some pretty horrible positions. Best I could get, in all of 1 minute thinking, was this:
15 forced half moves.

Sadly illegal, but more attractive than my last one when you play it through.
Still only 15 half moves. Ends in checkmate.