In the gutter box enter a width for the gutter margin.
How to set a left gutter margin.
In word there are two parts to setting up your gutter.
If you enable mirror margins with a top gutter setting it will be changed to left making it useless for duplex documents.
Change the size for the top bottom left and right margins.
This feature is not available in word online.
First if you intend to have left and right hand pages you have to tell word that.
This margin setting will be used in each new document you create based on that template.
In the gutter position box click left or top.
This accounts for the binding area.
Select set as default to set the new margin settings as the default for the current template.
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.
The purpose is to modify the horizontal margin to allow for the binding of documents within a folder.
Select layout margins custom margins.
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.
The left gutter setting can be combined with mirror margins as shown below.
But in addition to that margin you also want a gutter.
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.
The gutter goes on the left side of right hand pages and on the right side of left hand pages.
Word typically allows you to choose whether to position the gutter margin of your document at the top or left of the page.