Why is the > (or greater-than) character used to precede lines that are quoted from a previous message or forum post? I set out to find out for Meh.com in this article. I confess the ending is a bit of anti-climax: after consulting with the people who, in the early 1980s, wrote Usenet news-reader and distribution software and were involved with the email programs in common use, we pinpointed a particular year and even portion of year — but no one can remember being the person who put it into effect! I hope to one day track this down further.
This article may wind up in the book, as it's on topic, but because I don't have a definitive answer, I'm not yet sure if it'll make the cut. I may work it in in a reduced form.