If I think of AP as governing “fast” content (newspapers, online articles) and Chicago as governing “slow” content (books, some periodicals), then it helps to clarify how the reality of their media dictates style.
Main differences in concerns and goals:
- Layout: With newspaper columns (and online articles), you cannot always control where the break comes at the end of the line, whereas book and magazine publishers can fiddle with kerning, tracking, horizontal scaling, and soft returns, like, forever. Why this matters: Spaces around an em dash will allow it to break across two lines instead of dragging the words before and after it to the next line. Graphic designers working on a magazine article can, instead, manually insert a break after the dash and take their time making things look purdy.
- Deadlines: Most dailies and many weeklies are constantly under deadline. Having several nuanced style options to choose from will make the editors kill themselves, and that level of clarity is seen as excessive. The other camp, however, has more time to clarify a thought and nitpick its presentation.
- Compatibility: Ever get an e-mail with weird characters throughout? For copy to travel well—say, from final draft to wire to computer to publication—it must stay intact through its incarnations, with all intended letters, spaces, and punctuation in place. This places a premium on plainness, such as favoring characters over attributes (e.g., quote marks vs. italics). Non-journalistic content generally travels between fewer points and in the same form.
Maybe I just made all that up, but it sounds good, right? To me, it explains a whole lot, and otherwise kooky style rules come out looking quite reasonable when given the right context. (That's me being positive.)
I'm toying with the idea of having a little prize for people who help me catch typos on this blog or have awesome suggestions on how to make articles better. Stay tuned!