First, let's look at the basics. Let's say you have a print book that is 5,25" wide by 8" tall. You look at it on your desk and you can measure it, that it's 5.25" x 8" in size.
Now, open up the book. Try measuring the amount of the page that you can see, to read from. This is going to be a smaller amount, because a portion of each page is being used to hold the book together. It's where the glue and the binding are holding the pages together. Not only do you have to account for that actual glued part, but you also have to account for the words that are in near that area. It would be hard for a reader to try to read the words that are down in that spine, at an angle.
So in this book example, my book Seeking the Truth, while each page physically is 5.25" wide, in terms of readability there is only 4.75" visible. And as mentioned you wouldn't want your words up against even that visible area on the left, because it would be hard to read in there.
Here's another look at the book's spine, to show what is happening.
So what you do is you set a GUTTER for the book. The gutter is the amount of the book that you "lose" to the binding process. So yes, you have a half inch margin on the left and the right, to have white space to keep your book easy to read. But in ADDITION to that margin you also want a gutter. The gutter goes on the left side of right-hand pages, and on the right side of left-hand pages. This accounts for the binding area.
In Word, there are two parts to setting up your gutter.
First, if you intend to have left and right hand pages, you have to tell Word that. Otherwise it'll think that your intention is to publish only right hand pages, leaving the left ones blank. This is done in doctoral thesis papers sometimes, for example. So you would want to go into your header / footer area and ensure that you have it set to have both odd and even pages.
Once that's done, go into the margins area. There are spots there for you to set up your left margin, your right margin, and your gutter. A standard is to set the left and right margins at a half inch each, and to then have a quarter inch gutter. This particular book happened to be fairly thick (356 pages) so I set the gutter to 0.3 inches. That is what you see in the photo.
So if you're doing a physical book, make sure you understand what a gutter is, and that you set it up!
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