One of the basic criteria for choosing a gauge, appropriate for measuring a certain characteristic, is applying the "1 to 10" rule. It states that the effective resolution of a measuring device should be at least one class higher (that is with resolution 10 times higher) than the tolerance or the variation of the proces. An inquiring engineer may ask where this requirement is coming from, why the word "or" and which of the two values should he use? The answer becomes quite simple, when we explain what the purpose of a measuring process is and how the obtained measuring data influence a statistical evaluation of the process.