Simplicity - the art of maximizing the amount of work not done -- is essential

Keep It Simple
* By Doing Simple Things First 
* By Avoiding Unnecessaries 

Expressed as XP Principles
* DoTheSimplestThingThatCouldPossiblyWork
* YouArentGonnaNeedIt

Also inherent in:
* AgileModeling
* AskWhy
* CrcCard''''''s

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AgilePrinciples
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See - http://www.agilemanifesto.org/principles.html 
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CategoryAgileMethodology