There is much debate and confusion about the use of the passive voice in texts in general, and in computer manuals in particular. For example, it is often stated that the passive should be avoided, but on the other hand, it may also have a clear function in a text. The aim of this article is to provide clarity by presenting a straightforward principle for the use of the passive voice in computer manuals. This “alternation principle,” in which active voice is used for user actions and the passive voice for automatic computer, is backed by results from recent functional and cognitive linguistic research. It is illustrated by means of fragments from several computer manuals, including some (apparent) counter-examples.