The pages have to bend out from the spine.
Explain inside outside and gutter margins.
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.
In practice there is no difference between the gutter margin and the inside margin since both refer to exactly the same thing.
However one of your margins will need to be bigger than the others and that is the inside margin otherwise known as the gutter.
In a facing pages layout word refers to this type of layout as mirror margins the gutter margin is on the very inside of both pages.
Top bottom and outside margins and a 1 25 inside margin or a 1 margin and a 0 25 gutter on your portrait pages.
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.
By convention landscape pages whether recto or verso are oriented with the top of the page to the left.
To make it easier for the reader to read the text you should prevent the type from hiding in the gutter.
The gutter margin is a typographical term used to designate an additional margin added to a page layout to compensate for the part of the paper made unusable by the binding process.
Inside margin is in big way affected by the way the publication is bound and by the thickness of the publication.
Also keep in mind that prepress imaging at the printer or pod vendor is going to also account for creep in the thickness of the book by moving page images away from the spine to account for the thickness of folded signatures.
This creates a gutter where the two pages join.
This means that the top of the page will alternate from outside on verso pages to inside.
Outside or for edge margin.