In 1984, Bob Scheifler and Jim Gettys set out the early principles of X:
- Do not add new functionality unless an implementor cannot complete a real application without it.
- It is as important to decide what a system is not as to decide what it is. Do not serve all the world’s needs; rather, make the system extensible so that additional needs can be met in an upwardly compatible fashion.
- The only thing worse than generalizing from one example is generalizing from no examples at all.
- If a problem is not completely understood, it is probably best to provide no solution at all.
- If you can get 90 percent of the desired effect for 10 percent of the work, use the simpler solution. (See also Worse is better.
- Isolate complexity as much as possible.
- Provide mechanism rather than policy. In particular, place user interface policy in the clients’ hands.
The first principle was modified during the design of X11 to: “Do not add new functionality unless you know of some real application that will require it.”
X has largely kept to these principles since. The reference implementation is developed with a view to extension and improvement of the implementation, whilst remaining almost entirely compatible with the original 1987 protocol.