[This blog was inspired by a question in Quora]

V model is a conceptual model between design/requirements/needs and confirmation of those needs in the system. In waterfall, it is interpreted, unfortunately, as a staged model, where each stage is completed before the next step. For example, we want to consider user needs and business requirements as a whole before going into smaller details. On the other side of the V we have the verification/validation steps to confirm that the system meets the identified capabilities. First we go down the left arm of the V, and then climb back up the right arm.

This is a logical model - think of the bigger picture before smaller details, and consider how to confirm the system works accordingly at the different stages. But in the way the waterfall approach uses it, there is a huuuuge assumption behind the staged model - that the identified requirements at each level DO NOT CHANGE during the deeper V stages. A lot of people using waterfall approaches do not seem to understand how critical that assumption is, and how huge the impact is to the successful use of the model.