Conway's Law

From Wikipedia:  

Conway's law describes the link between communication structure of organizations and the systems they design. It is named after the computer scientist and programmer Melvin Conway, who introduced the idea in 1967.  His original wording was:

[O]rganizations which design systems (in the broad sense used here) are constrained to produce designs which are copies of the communication structures of these organizations.

— Melvin E. Conway, How Do Committees Invent?

The law is based on the reasoning that in order for a product to function, the authors and designers of its component parts must communicate with each other in order to ensure compatibility between the components. Therefore, the technical structure of a system will reflect the social boundaries of the organizations that produced it, across which communication is more difficult. In colloquial terms, it means complex products end up "shaped like" the organizational structure they are designed in or designed for. The law is applied primarily in the field of software architecture, though Conway directed it more broadly and its assumptions and conclusions apply to most technical fields.

Later they have a little more clear explanation.

Nigel Bevan stated in a 1997 paper, regarding usability issues in websites: "Organizations often produce web sites with a content and structure which mirrors the internal concerns of the organization rather than the needs of the users of the site."

Evidence in support of Conway's law has been published by a team of Massachusetts Institute of Technology (MIT) and Harvard Business School researchers who, using "the mirroring hypothesis" as an equivalent term for Conway's law, found "strong evidence to support the mirroring hypothesis", and that the "product developed by the loosely-coupled organization is significantly more modular than the product from the tightly-coupled organization". The authors highlight the impact of "organizational design decisions on the technical structure of the artifacts that these organizations subsequently develop".