‘Commonplace’ now mostly means ordinary, unremarkable — but the word started as the name of a very specific kind of notebook. A commonplace book was a personal anthology: passages copied out by hand from whatever you’d read, organized under headings you invented yourself — Friendship, Death, Kings, Anger — so that years later you could look up ‘Anger’ and find every good sentence about anger you’d ever come across, regardless of which book it originally lived in.
John Locke published an indexing method for keeping one in 1685, which tells you how seriously the practice was taken — this wasn’t idle scrapbooking, it was information retrieval technology in an age before search. The headings were the search terms. The whole point was that a good commonplace book let you retrieve an idea faster than you could re-find it in the original book.
I named the site’s citation map after this practice rather than after anything more contemporary — a Zettelkasten, a graph database — because the older word carries something the newer ones don’t: the assumption that the value of a passage is partly in where else it connects to, not just in the book you copied it from. See this note for the related habit of writing in the margin rather than copying out; the two practices fed each other constantly in the same reader’s hands.
The page here isn’t a copy of quotations, obviously — it’s a map of which of my own posts cite which others. But the instinct is the same one Locke was trying to serve: don’t trust memory to reconstruct the connections. Write them down where you can find them again.