The type area is the result of width and height of the typesetting. It must be set so that the front (recto) and side (verso) of a page are congruent.
The page number, also called pagination, is not part of the type area!
The type area determines the extent and appearance of a print work.
How to accurately lay down the type area:
The type setter defines the width of the typeset, which in turn determines its height. The following ways are available to do that:
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type width : paper width = 3 : 4
Resulting page margins:
Inner margin: 2 parts
Head margin: 2 1/2 parts
Outer margin: 3 parts
Foot margin: 4-5 parts
type width : paper width = 2 : 3
Resulting page margins:
Inner margin: 2 parts
Head margin: 3 parts
Outer margin: 4 parts
Foot margin: 5-6 parts
type width : paper width = 5 : 8 (Golden section)
Resulting page margins:
Inner margin: 2 parts
Head margin: 3 parts
Outer margin: 5 parts
Foot margin: 8 parts
When you define defining a type face, keep in mind that you shouldn't just do the mathematics, but you may also need to make visual corrections.