What is the explanation for the point system for type?

Answer 1

In the American point system:

• 1 pica = exactly 0.1660 inch (versus 0.1666 = 1/ 6 inch for the DTP-pica) = 4.216 400 mm.
• 1 point = 1/ 12 traditional pica = exactly 0.013 83 inch = 0.351 36 mm.

The American point system has been used in the USA, Britain and many other countries including Japan.

Today, digital printing and display devices and page layout software use a unit that is different from these traditional typographic units. On many digital printing systems (desk-top publishing systems in particular), the following equations are applicable (with exceptions).

• 1 pica = 1/ 6 inch (British/American inch of today) = 4.233 mm.
• 1 point = 1/ 12 pica = 1⁄72 inch = 127⁄360 mm = 0.3527 mm.

en.wikipedia.org/wiki/Typographic_unit