**Architecture decisions** form the constraints of the system and direct the development teams on what is and what isn’t allowed. A **design principle** differs from an architecture decision in that a design principle is a guideline rather than a hard-and-fast rule **What is variance in architecture?** If a particular architecture decision cannot be implemented in one part of the system due to some condition or other constraint, that decision (or rule) can be broken through something called a variance. Most organizations have variance models that are used by an architecture review board (ARB) or chief architect.