Furthermore, Roman and Italic are two entirely different styles of characters. For highlighting purposes, printers started using Italic within Roman text, and eventually pairs of Roman and Italic fonts became identified with each other, so that now typefaces generally include both of them.
(We may observe that the upcoming use of white space in writing in order to structure text roughly coincides with the introduction of zero into the number system. We may also observe that the use of zero was already implied by the Roman abacus, but didn't propagate to writing numbers. This may be seen in the context of Roman writing dismissing interpunctuations originally imported from the Greek and eventually also dismissing spaces between words, as it was considered important for a reader to immerse in the text in order to reveal its meaning. This approach to writing and reading may also shed some light on the horror vacui that may be observed in medieval writing. For an opposite approach we may consider the Summa Theologiae, breaking down the text in structure and form so it provides a quick and easy orientation for where in the text we currently are. This approach brings also some new characters, like "¶" to mark the beginning of a paragraph. – Historically, there's been an ongoing discussion whether to favor legibility or rather illegibility in writing and letter forms, which is linked to the question, whether a text should reveal its meaning quickly, or rather resist a cursory reading.)