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View | Layout | Portrait (Landscape) menu command

As you already know xlCompare divide it's working area into 2 panes to display compared data.

In some worksheets it is more suitable to have them aligned vertically, one file on the left and another one on the right. This is default Portrait layout. You can see it on this screenshot:

xlCompare - Portrait layout

But, if you are working with worksheets, which have many columns, you can use Landscape layout:

xlCompare - Landscape layout

In this case working area is divided into the top and bottom panes, and worksheets are shown one under one.