Welcome to CSR Nation
What a great list of resources! Thank you for compiling this.
I'm brand new to this site, and joined after seeing the wealth of information here. I found CSR Nation while searching for an answer to a punctuation question. I'm hoping someone here can offer their input.
I have been a proofreader of depositions for 14 years. On occasion, I have had to punctuate an interrupted thought where the last word before the em dash was incomplete. I learned (from what source, I can't remember!) that an en dash should not precede the em dash. Here's an example:
I went to the store on Fri- -- no, it was Saturday.
Aside from a source to justify my removal of the dash, my own thinking is that it would be the same as having a comma, period or semi-colon before the em dash.
Any bona fide rule about this would be greatly appreciated!