The technique of domain testing goes under several names, such as equivalence partitioning, boundary analysis, and category partitioning. And probably a few others that I’m either forgetting or never heard of. While this technique is a formal test design method, many testers will tend to do this technique somewhat intuitively. So what I want to do here is show the basis behind this intuition as well as a few simple working examples of how the technique is applied — with an emphasis on how intuition can fail.