The human being has always needed to measure distances, periods of time, quantities, weights...
When we measure something, we compare it to the model we have taken as the unit.
Measuring distance in steps and palms is easy, but these are not very accurate, because they vary from one person to the other.
A natural measurement unit does not exist; the measurement units we use have been established by men.
In the past a variety of different measurement units has been used, each region having its own interpretation of these units. This lead to misunderstandings since there was no official model, that is, the same unit varied in length from one region to the other.
These old units did not have multiples and submultiples of 10, which made measuring and calculating rather complicated.
Many town councils and goverments tried to establish an official measurement unit, but it wasn't until the French Revolution that a precise and easy handling measurement system was established. It was named decimal metric system, which is the one we used in Spain and in most countries, as well.

The metric system was not established from the beginning as we know it now, but it has been improved throughout its two centuries of existence. Nowadays it is called The International System of Units and it defines seven fundamental units: metre, kilogram, second, ampere, kelvin, mole and candela.

Most people around the world use metric units but, in Britain, many people still use imperial units. The imperial units still commonly in use are:

Using the same model of measurement is necessary to buy and sell things and, when studying physical phenomena and establishing the relation between them, you need to measure them with accuracy.