I can’t emphasize the importance of rule-5 enough.

I learnt about rule-5 through experience before I had heard it was a rule.

I used to do tech due diligence for acquisition of companies. I had a very short time, about a day. I hit upon a great time saver idea of asking them to show their DB schema and explain it. It turned out to be surprisingly effective. Once I understood the scheme most of the architecture explained itself.

Now I apply the same principle while designing a system.