Dimensional metrology uses terms such as accuracy, repeatability, and precision, just to name a few. Measurement is a comparison process whereby an unknown quantity is compared with a known quantity or standard. Absolute or direct measurement instruments include rules, calipers, height gages, and micrometers. Comparative measurements have the advantage of being considerably more accurate than direct measurements. Comparison measurement instruments include dial indicators, test indicators, and optical comparators. Coordinate measurement is a two or three-dimensional process that determines the position of holes, surfaces, centerlines, and slopes. Measurement errors exist regardless of the instrument’s precision or the skill of the operator using the instrument.