Thriller Guy thought he'd step in here because he knows A. Appel doesn't have the stones to properly address this important topic: Those dinosaurs who are still putting two spaces after a period.
Berkeley Breathed has done a number of strips on his web page devoted to this topic. Over the years, TG has asked on these pages that those of you who are still doing this, please stop. As always, you're not listening. TG notes that for each of the strips Breathed has done on the topic, especially the Sunday strips, there have been 40 to 50,000 likes, comments, and shares. Yes, that is the correct number. And most of the comments are of the variety, "You can have my two spaces when you pry them from my cold, dead hands." That's a lot of people who are delusional.
People, writers, who do this should just tack a message at the beginning of their manuscripts that speaks directly to the YOUNG editors, agents and readers who are reviewing their query letters, partial and complete manuscripts or any other communications that has come across their desk, via email, snailmail or in any other written form. That message says, "I am old. I am clinging to outdated rules. My work will be old fashioned. My ideas are unoriginal. I am a loser."
Go ahead, howl with indignation. Gnash your self-righteous teeth. If you'd like a kinder, longer explanation why you have to stop doing this, read this web article.
Not long ago, one of the agent sites TG likes to look at on occasion ran a piece written by an agent about how he judges manuscripts. The sentence read something like this, "When I open a manuscript, and I see that the writer is using two spaces after a period, I throw it in the trash." You think that's kind of harsh? If you've got a slush pile of fifty manuscripts to work your way through, you'll use any shortcut you can find to winnow out the "bad" ones. Agents and editors don't have the time to read all the submissions that flood in on them every day. It's your job to write the best book you can write, and present it in the best possible manner.
Years ago, TG used to do the two spaces thing. His son, TG Junior, laughed and gave him the scoop on the practice. TG quit. It took about a day to replace the habit with a single space. TG wishes he could say from that moment on his manuscripts found instant homes and the money poured in. They didn't, and it didn't. But you know what?
His writing no longer made him look like an old fool.
So keep it up if you think you must, there are thousands like you. You know who they are, they're called the unpublished.