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.
In the gutter position box click left or top.
In word there are two parts to setting up your 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.
The left gutter setting can be combined with mirror margins as shown below.
Select layout margins custom margins.
Gutter margins are adjustments made to horizontal positioning in addition to the standard left margin setting.
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.
But in addition to that margin you also want a gutter.
Select set as default to set the new margin settings as the default for the current template.
This margin setting will be used in each new document you create based on that template.
If you enable mirror margins with a top gutter setting it will be changed to left making it useless for duplex documents.
In the gutter box enter a width for the gutter margin.
The gutter position box is not available and determined automatically when you use the mirror margins 2 pages per sheet or book fold option.
This feature is not available in word online.
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.
In effect a 1 inside margin combined with a 1 gutter is the same as a 2 inside margin.
Type numbers indicating the width of your margins in the top bottom left and right fields.
First if you intend to have left and right hand pages you have to tell word that.
Change the size for the top bottom left and right margins.
The purpose is to modify the horizontal margin to allow for the binding of documents within a folder.