To Function or Not to Function, That is the Question

Question:
What is the difference between a functional and non-functional requirement?

Answer:
If a requirement uses a verb to describe the feature then it is functional; What will the system *do*? If a requirement uses an adverb or adjective to describe the feature, 'how' will it do whatever, then it is non-functional.

I've almost always been interested in the importance of language in discussing requirements. There are two ways to listen to your customer, literally and figuratively. That the customer has difficulty describing their requirements has been a problem in the software field for so long that it's now a cliche. What some analysts have done in to start reading between the lines to capture what the customer is "really" saying. There can be some benefit to this figurative approach but I would always to to listen to them literally. In many cases this meant inconsistent or confusing descriptions, the key was to realize that those questionable points were just that, points to question. In getting clarification about those requirements, we can get a more detailed understanding of the problem domain. If their choice of words or phrases contradict, ask them about it.

Sometimes we need to listen figuratively or "between the lines" to get the overall picture but listening literally can be indispensable for uncovering the things you need to dig into for more details.