user29825: There are positions where no matter what legal move black or white does during this sequence of moves, an specific player, will always be mated after X moves (sometimes earlier, but after X moves its a certain thing).
What is the position with biggest value for X?

user15543: The longest known sequence is 22 plies long.
[Title "Alexey Khanyan, Tim Krabbe's Website Diary Entry #267 2008, Mate In 11"]
[FEN "4Q2Q/4r3/4n1n1/1bbK1krn/RR1RR1RR/2qn1R1n/4n1nN/Q3Q3 b - - 0 1"]
[startflipped ""]

1... Ng2f4+ 2. Rfxf4+ N2xf4+ 3. Rgxf4+ Nh3xf4+ 4. Rhxf4+ Ndxf4+ 5. Rxf4+ Nhxf4+ 6. Rxf4+ Ngxf4+ 7. Rxf4+ Nxf4+ 8. Rxf4+ Kxf4+ 9. Qee5+ Qxe5+ 10. Qaxe5+ Rgxe5+ 11. Qxe5+ Rxe5+ 12. Qxe5#

Source: Diary Entry #267

29825: <p>There are positions where no matter what legal move black or white does during this sequence of moves, an specific player, will always be mated after X moves (sometimes earlier, but after X moves its a certain thing).</p>
<p>What is the position with biggest value for X?</p>
 15543: <p>The longest known sequence is 22 plies long.</p>
<pre><code>[Title &quot;Alexey Khanyan, Tim Krabbe's Website Diary Entry #267 2008, Mate In 11&quot;]
[FEN &quot;4Q2Q/4r3/4n1n1/1bbK1krn/RR1RR1RR/2qn1R1n/4n1nN/Q3Q3 b - - 0 1&quot;]
[startflipped &quot;&quot;]

1... Ng2f4+ 2. Rfxf4+ N2xf4+ 3. Rgxf4+ Nh3xf4+ 4. Rhxf4+ Ndxf4+ 5. Rxf4+ Nhxf4+ 6. Rxf4+ Ngxf4+ 7. Rxf4+ Nxf4+ 8. Rxf4+ Kxf4+ 9. Qee5+ Qxe5+ 10. Qaxe5+ Rgxe5+ 11. Qxe5+ Rxe5+ 12. Qxe5#
</code></pre>
<p>Source: <a href="https://timkr.home.xs4all.nl/chess2/diary_14.htm" rel="noreferrer">Diary Entry #267</a></p>
