Abbe’s principle relates to accuracy when measuring dimensions. The principle is also an important guideline for designing measuring instruments. This principle states that, "In order to improve measurement accuracy, the measurement target and the scale of the measuring instrument must be placed in a collinear fashion in the measurement direction."
When applied to actual measuring instruments, the principle implies that while the scale and the measurement position are collinear for outside micrometers, the scale and the measurement position are at a distance from each other for hand calipers. In other words, outside micrometers follow Abbe’s principle, while hand calipers do not. Therefore, it can be said that outside micrometers have a higher measurement accuracy.