The double-slit (gedanken-) experiment is the most famous experiment in quantum theory (QT). The explanation of the strange behavior of the electron in this experiment is used as a key example in QT in general. The description of the experiment includes a rationalization of when in quantum mechanics interference occurs and (most importantly) when it “collapses”. The aforementioned rule, here called the “interference collapse rule”, is contained in almost all textbooks of QT with only slight variations. However, this rule makes sense only with additional assumptions which apparently are not generally agreed upon among physicists. The paper proposes an improved interference collapse rule that connects the interference collapse to the QT measurement and a functional interpretation of QT measurement.