“Organizations which design systems are constrained to produce designs which are copies of the communication structures of these organizations.”
— Conway’s Law
System here is intended in a broad sense, can be anything from software to processes. This is a very important rule that every person should know, not just managers. It applies to any work done by any group of people, regardless of the field.
It tells a lot of good things.

  1. It explains why certain systems developed in a certain way: just look at the people.
  2. It tells you that you can’t change the architecture of a software if you don’t change at the same time how the people working on it are organized.
  3. It tells you to hire consultancies that work in a way you’d like to work, not companies that you don’t want to resemble.
  4. It hints you on how to structure new projects, starting by thinking how groups are organized.
  5. It tells you to not buy a software that implies a different practice than the one your team is comfortable to work with.
The beauty of this is that can work well on things as structured as engineering and as loose as organizing a party.
In a very basic form this means that the way you form the groups (group A: do this, group B, do this other part) will match the way things will be organized in the end.