Arrange the checks a court makes under the principle of legality, in the order they arise.

1. A written statute of the required rank described the conduct before it was done
2. The description covers this conduct without being extended by analogy
3. The statute itself fixes the penalty to be imposed
4. The penalty is imposed by the competent court in the process laid down
5. The sentence is served in the manner laid down by law and regulation

Read downwards, the five checks are the four guarantees with the requirements of the statute folded into the first three. That is the useful way to hold them: the guarantees describe the stages at which legality bites, and the requirements describe the quality the statute must have for the first two stages to mean anything.