p. v. pp.

Quoting Wikipedia:

http://en.wikipedia.org/wiki/Abbreviation

In Latin, and continuing to the derivative forms in European languages as well as English, single-letter abbreviations had the plural being a doubling of the letter for note-taking. Most of these deal with writing and publishing.

The rest of the wp article discusses the use of the final period as well (cf "History" section).

I remember learning this in high school, when I was taking Latin. The textbook had a picture of a coin that bore the face of some monarch. The coin identified this person as the reigning emperor of "BRITTANNIA," if I remember the spelling correctly. According to the book, the double T to pluralize the word "Britannia" --- which in this case referred to the British Isles --- in the same way that "pp" means pages. Don't know why my brain held onto this all these years, but it did.

I have always used p. for one page and pp. for two or more pages.

And the plural for the barrels of a carburetor is bbl.