XP has four the most principles as showed below:

1, Simplicity: for the requirements are always inevitable, so, the more simple to build simple model for customer, the more quick customer's feedback will be received, make sure we will go the wrong way or make some even good thing but customer don't want.

2, Communication: programming communicate with each other can through unit case even better than source code; about pair programming, one for coding, one for testing or additional design, also they swap often. We also need a on-site customer to answer our questions.

3, Feedback: for customer, the feedback is very important, it will give new features or some thing mistakes about customer's requirments, about two weeks once, customer will get the new version of product and feedback will be given later days, also the code can feedback to programmers by some errors, and the unit test is a very important way to implement that.

4, Courage: the Manager has enough courage to do pair programming - it seems like productivity will drop 50%.

 

to be continue...