In the course of time, there were different measures in different countries. For example, there were the francogerman Fournier System and the Inch-Pica System. Today, the DTP System is used almost everywhere.

Here's some more information on these systems:

 

The Fournier System


The typographic point was developed by Pierre Simon Fournier. He chose 12 Cicero as the initial dimension. 12 Cicero are 2 inches of the then common measure in ... 

The Didot System


The Didot System was named after its inventor, the french type founder Firmin Didot (1761 - 1836). It was invented at the end of the 18th century. 

The Pica System


In the USA, the Fournier System was further used. Later on, it was merged with the british Caslon System and formed the Pica-Point System.