Key

In its standard use in directmate problems, the key is the unique White first move that begins the correct solution. In a directmate, White forces mate against Black's resistance. After the key, the stipulation, meaning the task stated by the problem, must be achievable against every required legal Black defense.

A key does not have to be a check, a capture, or the most forcing-looking move. It can be a quiet move. What matters is not whether it looks dramatic, but whether it correctly organizes every variation that belongs to the solution.

The key is easiest to understand by comparing it with other first moves. A try appears to work but is defeated by a refutation. A cook is an unintended second first move that also solves the task and therefore damages soundness. Set play, by contrast, describes variations prepared before the key is made.