Q. In a sentence, a colon should always be preceded by an independent clause. Why doesn’t the Chicago Manual state this explicitly? All your examples follow the principle. Why doesn’t the manual just say that the introductory clause has to be independent?

A. Because we’re a bunch of spineless and ineffectual prevaricators? Or because there are times when a colon need not be preceded by an independent clause? A case in point: this one.