Weber on Italic
Hendrik Weber, Italic—What Gives Typography Its Emphasis, (p.19):
In the meantime, typography has performed a balancing act by combining modern and traditional forms. Italics fulfil a typographic mission. They have to stand out clearly from the typeface used for the main text, without invalidating it. For this reason, italic forms today are not only dependent on the zeitgeist but in particular on their upright counterpart’s design.
He went on:
Upright and italic typefaces mutually depend on each other. Without upright typefaces, there would be no italics—and without italics, the options a designer would have when laying out a text would be rather limited. A modern typeface should have at least one italic, as a minimum. The number of italics was also connected with the increasing formal variety of upright text typefaces. This adaptive behaviour to any typeface construction testifies to italics’ quality. At the same time, it is also the reason italics are difficult to classify into a particular category or group.