Q. As a proofreader, I always mark a bad break when a line ends with an em dash and then a divided word:
This part of the street was relatively modest—boast-
ing a bank.
But I can’t find anything in CMOS that actually says this is necessary. Am I missing it? I also work for one publisher who considers it a bad break when an em dash appears after the portion of the word carried over:
This part of the street was relatively mod-
est—boasting a bank.
Is that rule any more or less valid than the preceding one?
A. Chicago’s guidelines for proofreading word division (2.112, 17th ed.) don’t prohibit such breaks, pointing out that the cure might be worse than the disease, resulting in a squished or loose line.