Below are a batch of ampersands, arranged more or less chronologically, according to when the original model was created. (Can you identify the faces? They’re all ordinary faces that are among the “standard” repertoire of text faces.) On the left are roman versions and on the right italics.
The ampersand is a kind of ligature. It represents the letters e and t, spelling et, the latin word for and. Ampersands, like all ligatures, can provide a bit of typographic flair (ligatures also prevent unfortunate combinations such as when the dot of an i might crash against the roof of an f, for example). In they days of metal type, however, ligatures performed an additional function — they were timesavers. A typical book these days might contain about a hundred thousand words, each of which represents roughly six characters in English. Over the course of setting six hundred thousand characters, the savings represented by letter combinations could add up.
With the advent of the typewriter, nonmechanical typesetting, and desktop publishing, the use of ligatures declined (perhaps, with opentype and other recent developments, they are making a comeback). But the ampersand has remained popular. If you look at these examples you can see that with early faces there is a big difference between the roman and italic versions. Through example number five I would throw out the roman ampersands and use the italic versions exclusively.
In examples six and seven we see something different. The pen metaphor is no longer dominant in these typefaces, which instead reflect the model of drafting. In these faces there is little or no advantage to the italic. In such modern faces, beginning around the Romantic period of the early nineteenth century, you may well be better off using the roman ampersand entirely. With sanserif faces such as example 8, whatever creates the best color is probably best — generally the ampersand should match the slant of the surround characters.
The examples are 1. Centaur (Venice, 15th c.; Centaur MT Std), 2. Garamond (France, 16th c.; Adobe Garamond Pro), 3. Janson (Netherlands, 17th c.; Janson Text Lt Std), 4. Caslon (England, 18th c.; Adobe Caslon Pro), 5. Baskerville (England, 18th c.; ITC New Baskerville Std), 6. Bodoni (Italy, 18th-19th c.; Bodoni MT), 7. Times (England, 20th c.; Times New Roman PS MT), 8. Univers (Switzerland, 20th c.; Univers LT Std). The figs, by the way, are all Bembo Book MT Pro.