We define a "tester" as someone who not only tests but who has QA and development skills.
Our use of the term "tester" involves both QA and QC activities, and we drop the distinction between the two and encompass both in the integrated testing activities of XP.
A tale of remodeling in which the contractor did not appreciate how the presence of a doorway in the plans should imply a door illustrates how customers' assumptions get missed in the development process.
A tester makes a contribution by viewing the system from the standpoint of the customer but with the grasp of details, possibilities, and constraints of the developer, avoiding the "missing door" syndrome.
A tester provides a skeptical outlook on the correctness of the system that helps keep the team from overlooking things that will make the customer unhappy.