Q. Does CMOS weigh in on whether email subject lines should be capitalized in sentence case or title case?
A. Great question! CMOS doesn’t cover email subject lines, but if we did we’d make a distinction between personal email messages, on the one hand, and formal announcements or mailings, on the other.
For personal messages (even if they’re related to work), sentence case would usually be appropriate. Not only will sentence case come across as less formal than title case, but it will save the sender the trouble of determining which words get capitalized and which do not.
Subject: Last year’s Halloween party
Formal messages sent out by an organization, on the other hand, may be subject to the same editorial scrutiny that a press release or similar document might get. In that case, Chicago’s version of title case would be appropriate for organizations that otherwise follow Chicago style (see paragraph 8.160 for the principles of title case).
Subject: Supporting Our Community Through the Coronavirus Crisis
Note that not all styles apply title case in the same way. For example, whereas Chicago capitalizes prepositions of more than four letters (including “through” in the example above), APA and AP capitalize all words that are longer than three letters. AP further distinguishes between titles mentioned in text and headlines for news stories, prescribing sentence case for the latter. So be sure to review your organization’s style before applying it to formal email subject lines.