When it's a bookshelf speaker laying on its side.
Just an FYI to anybody who can't fit a traditional center speaker in their TV stand/credenza. All three sections of mine are designed for 17" equipment, not the larger width of almost any decent center speaker. So...
I am using a single Monitor 40 laying on it's side. And, it sounds great. In fact, it's specs are better than the CS1, CS2, or CS10, and on par with the CS20. And, it matches my Monitor 70's seamlessly across the front.
Of course, you have to buy a pair. But, at $129 for the pair, it's still cheaper than one CS20. And, you can always sell the extra one to somebody else with the same space constraints. (I plan on listing mine in the classifieds, as soon as I reach 25 posts.)
I was doing the same thing with my last set of speakers, as well. But, because of the grill design, it looked like a bookshelf speaker on its side. The Monitor 40, otoh, looks great on its side. Only if you look closely at the logo, is it noticeable.
I'm sure I'm not the first to do this. But, I haven't seen it mentioned here. So, I thought I'd mention it. I hope it helps someone.