In such a case type a number in gutter that will allow enough room for the binding and use the drop down to indicate whether the binding will.
How to set a left gutter margin.
Select set as default to set the new margin settings as the default for the current template.
If you have previously set your document to the mirror margins book fold or two pages per sheet layout options word will automatically set the gutter margins to a position that matches the chosen layout.
Gutter margins are adjustments made to horizontal positioning in addition to the standard left margin setting.
First if you intend to have left and right hand pages you have to tell word that.
In the gutter position box click left or top.
The left gutter setting can be combined with mirror margins as shown below.
Change the size for the top bottom left and right margins.
If you enable mirror margins with a top gutter setting it will be changed to left making it useless for duplex documents.
Only adjust the gutter margin if you intend to use the document in a bound format like a book or report and you need space for the binding.
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.
Word typically allows you to choose whether to position the gutter margin of your document at the top or left of the page.
This feature is not available in word online.
The gutter position box is not available and determined automatically when you use the mirror margins 2 pages per sheet or book fold option.
Select layout margins custom margins.
In effect a 1 inside margin combined with a 1 gutter is the same as a 2 inside margin.
In the gutter box enter a width for the gutter margin.
This margin setting will be used in each new document you create based on that template.
But in addition to that margin you also want a gutter.
The adjustment is added to the left margin for the front face but subtracted from the left margin for the rear face of duplex pages.