Typography and Page Layout: The Printers' Point System

In the year 1898 the English typefounders, as a body, adopted a system (which had been in use in America since 1878) of casting their types to a certain fixed standard. That standard was the American pica, 83 of which equalled 35 centimetres. The pica, which measured 4.21mm, was divided into 12 equal parts called 'points', which makes the size of a point approximately 0.35 mm.

Magnik, John. Typography First. Design>Document Design>Typography>History