When aligning rotating machinery it is imperative that you obtain accurate shaft positional measurements. It is very easy and very common to make mistakes when taking these measurements so this tutorial will illustrate what you have to be aware of when taking your alignment readings to insure that the measurements you obtain are accurate and repeatable. There is a mathematical pattern that occurs when taking readings at 90 degree intervals around a shaft. This pattern is called the Validity Rule. The Validity Rule will confirm that your measurements make sense but more importantly, can derive a missing measurement if you cannot rotate all the way around the shaft.