A type of
chess problem where black moves first and co-operates with white in being
checkmated. For example, the stipulation "helpmate in two" asks the solver to find a sequence of four moves (black, white, black, white) culminating in white checkmating black.
For example, here is the starting position for a helpmate in three by Sam Loyd: white has his king on d4, rook on g8 and bishop on f4, while black has his king on f5 and queen on h7:
+---+---+---+---+---+---+---+---+
| | | | | | |WR | |
+---+---+---+---+---+---+---+---+
| | | | | | | |BQ |
+---+---+---+---+---+---+---+---+
| | | | | | | | |
+---+---+---+---+---+---+---+---+
| | | | | |BK | | |
+---+---+---+---+---+---+---+---+
| | | |WK | |WB | | |
+---+---+---+---+---+---+---+---+
| | | | | | | | |
+---+---+---+---+---+---+---+---+
| | | | | | | | |
+---+---+---+---+---+---+---+---+
| | | | | | | | |
+---+---+---+---+---+---+---+---+
Here's a hint at the solution, printed backwards:
gnik sih sevom reven etihw dna gnik sih ylno sevom kcalb.