Type - The Graphic Design: Reference and Specification - page 24

m e a s u r i n g t y p e
Type is measured by points, a unit for measuring the height of type and vertical distance
between lines of type. A point measures .0138 of an inch or 3.515 mm. There are 72.27
points in an inch and 28.453 points in a millimeter.
8 point
10 point
18 point
24 point
36 point
48 point
60 point
72 point
The point system of measuring type goes back to the days of metal type when sizes were classified according to the metal slug
that held each character.