At each step of a development, use descriptions that are as formal as possible.

This has two benefits:

- Formal descriptions are more precise than informal ones, which forces a thorough understanding of the issues and questions.
- More powerful verification methods for formal descriptions exist than methods for informal ones, so there is more confidence in each stepâ€™s correctness.

The rule of maximum practical formality was articulated in the paper Correctness by Construction: Developing a Commercial Secure System (see the top of page 21 and the bottom of page 19)