I'd also like to know because with some fonts I find that italic does not stand out enough (for transparencies) and bold (or even semibold) is too agressive and the corresponding italic looks more pleasant. My intent is not double insistance. In a mathematical definition, the term that is being defined is normally put in italic.

2. Who invents the first bold typeface?

Do you mean a typeface bold in appearance or a typeface with (also a) bold style?
The first were used as a mean of emphasis and differentiation before the invention of the latter.

Do you mean a typeface bold in appearance or a typeface with (also a) bold style?

¶ I mean a typeface with a bold style. Was it already there during the French Renaissance?

¶ In the XIX th century, there were the fat faces by Thorne.

> Bold italic implies a double emphasis. Why?

There are situations where the copywriter will specify italic (titles of books, plays or albums, to emphasize a word in a sentence, etc). If that sentence is already in bold, you need bold italic.

Pretty much what JamesM said. If you need to italicise a word in a bolded paragraph, you need bold italic. Bold is occasionally used in text for "darker spot on the page that you see immediately" emphasis (I do it myself in emails to make them easier to parse, or when a new key term is introduced in an academic paper) but more often in serious typographic settings for entire sections, for instance in newspaper ledes. (And of course for headings, display type, in tables etc.)

http://www.typophile.com/node/31036