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.