Edge files are used for rendering frame borders, and are specified with the edgeFile attribute of a <Backdrop> tag, or the edgeFile element of a backdrop specification table.

An edge file is just an image file (obeying the normal WoW file size rules), which will then be split horizontally by the UI engine into 8 equally sized slices (The actual file dimensions shouldn't matter as long as the width is a multiple of 8).

The slices are placed as follows (going from left to right)

• Left edge
• Right edge
• Top edge (Rotated 90 degrees Counterclockwise)
• Bottom edge (Rotated 90 degrees Counterclockwise)
• Top Left corner
• Top Right corner
• Bottom Left corner
• Bottom Right corner

The frame border is then made by repeating the edge images in between the four corner images.

The EdgeSize parameter on the frame's backdrop indicates how wide the image will be stretched on to the frame.