On the site (http://reference.wolfram.com/language/ref/CellMargins.html) it says use "SHOW RULER". I cannot find Show Ruler anywhere in the Mathematica UI. One can select menu item Window -> Toolbar -> Ruler, and a ruler does appear on the top of the page. Moving the left angle bracket to "1" did create more space in the window open on the right, but had no effect on the window open on the left. What the requester (Michael McCain, Ventura College) wants is one setting that will make more space available on every window every time a new page is created. I think that is a great idea.

What does the little triangle approximately 2/3 of the way across the ruler signify? Mine does not move. Should it?

I think we first have to clarify what "Margins" means. It can be a bit ambiguous.

Do this mean WindowMargins? (http://reference.wolfram.com/language/ref/WindowMargins.html)

Or maybe you want to leave more space around the Notebook's cells with CellMargins? (http://reference.wolfram.com/language/ref/CellMargins.html)