The orientation of the thumbnail viewer.
If both the Rows property and the Columns property are each equal to the default value (0), this property will determine the orientation of the thumbnails in the viewer.
If the Rows * Columns value is greater than the number of the thumbnails in the viewer, this property will determine which property should be used.
For example: if Rows = 3, Columns = 3, the number of thumbnails (Count) is 6, and this property is set to Horizontal then the final view would be 2 rows x 3 coulmns.
Refer to Add(String,Int32,Int32)