Variation 1. position must be legal:
As there are at most 15 capturable white pieces this is a hard upper bound on the chain length.
This bound can
easily
be achieved.
Documentation:
[FEN "1N1k4/1QR2PPP/4P1n1/n2PN1r1/2P5/1P1BB3/P2R4/5K2 w - - 0 1"]
`
1.h8=Q+ Nxh8 2.g8=Q+ Rxg8 3.Bg5+ Rxg5 4.Nbc6+ Nxc6 5.Qb8+ Nxb8 6.Rd7+ Nxd7 7.f8=Q+ Nxf8 8.Nf7+ Nxf7 9.e7+ Kxe7 10.d6+ Kxd6 11.c5+ Kxc5 12.b4+ Kxb4 13.a3+ Kxa3 14.Ra2+ Kxa2 15.Bb1+ Kxb1 *
Variation 2. position need not be legal:
I can do
60
[FEN
[FEN "kqBQBQBQ/1RRRRRRR/BRRRRRRR/BRRRRRRR/BRRRRRRQ/QRRRRRRR/BRRRRRRR/QRRRRQRK w - - 0 1"] 1. Ra7+ Qxa7 2. Rb8+ Qxb8 3. Ra7+ Qxa7 4. Rb8+ Qxb8 5. Ra7+ Qxa7 6. Rb8+ Qxb8 7. Ra7+ Qxa7 8. Rb8+ Qxb8 9. Ra7+ Qxa7 10. Rb8+ Qxb8 11. Ra7+ Qxa7 12. Rb8+ Qxb8 13. Ra7+ Qxa7 14. Bab7+ Qxb7 15. Ra6+ Qxa6 16. Bb7+ Kxb7 17. Qb6+ Qxb6 18. Rc7+ Qxc7 19. Rb6+ Qxb6 20. Rc7+ Qxc7 21. Rb6+ Qxb6 22. Rc7+ Qxc7 23. Rb6+ Qxb6 24. Rc7+ Qxc7 25. Rb6+ Qxb6 26. Rc7+ Qxc7 27. Rb6+ Qxb6 28. Bac6+ Qxc6 29. Rb5+ Qxb5 30. Bc6+ Kxc6 31. Qac5+ Qxc5 32. Qd6+ Qxd6 33. Rc5+ Qxc5 34. Rd6+ Qxd6 35. Rc5+ Qxc5 36. Rd6+ Qxd6 37. Rc5+ Qxc5 38. Rd6+ Qxd6 39. Rc5+ Qxc5 40. Rd6+ Qxd6 41. Bgd5+ Qxd5 42. Re6+ Qxe6 43. Bd5+ Kxd5 44. Qhe5+ Qxe5 45. Qd4+ Qxd4 46. Re5+ Qxe5 47. Rd4+ Qxd4 48. Re5+ Qxe5 49. Rd4+ Qxd4 50. Re5+ Qxe5 51. Qe4+ Kxe4 52. Rf4+ Qxf4 53. Re3+ Qxe3 54. Rf4+ Qxf4 55. Re3+ Qxe3 56. Qf3+ Kxf3 57. Rf2+ Qxf2 58. Rg3+ Qxg3 59. Rf2+ Kxf2 60. Be1+ Kxe1
