It is commonly understood that specification languages are used to describe what is to be implemented while programming languages allow us to spell out how this is to be done. Hence, it is usually assumed that specification languages are not executable, in contrast to programming languages.

However, this approach is too simplistic. We will discuss this issue showing, in particular, examples of languages which changed their nature in time but our focus will be on the state of affairs in the early years of computer science and software engineering.