# 2 - System Architecture
Every system built by humans has an architecture. Systems such as cars, airplanes, mobile phone software are defined by a few key decisions that are made early in each program’s lifecycle. For example, early decisions in automobiles development, such as the mounting of the engine, drive a host of downstream decisions. Choosing to mount an engine transversely in a car has implications for the modularizing of the engine, gearbox, and drivetrain, as well as for the suspension and the passager compartment. The architecture of a system conveys a great deal about how the product is organized. In the design of complex systems, many of these early architectural decisions are made without full knowledge of the system’s scope. These early decision have enormous impact on the eventual design. They constrain the envelope of performance, they retract potential manufacturing sites, and so forth. As an example of gathering downstream information for upstream consumption, the overall width of some pro...