History of length measurement
Arbitrary Units of Measurement: Before the development of standard units of measurement, people used arbitrary units of measurement. Some of them are still in use at some places. Handspan, 
finger-length, cubit (length between the elbow and finger tips), foot-length and arms-length are some examples of arbitrary units of measurement.

Disadvantages of Arbitrary Units of Measurement: You have seen that arbitrary units are made up of lengths of different body parts. These lengths can vary from one person to another. This can create lot of confusion as there would be no uniformity in measurement. We need standard units of length and other physical quantities to obtain the same value for a measurement.