Digital BW, The Print

The problem with corners is that if you make them fit very snugly around a
print that is framed in a dry environment, the print will curl as it expands
when it is hung in a climate that is more humid.  Sometimes you can get
around this by having the bottom corners be less snug so that the print can
expand a bit without creating pressure, but this is only possible if the
overmat cuts into the image a bit.  If your overmat allows a border between
the image and the overmat, then you run the risk of the print shifting and
the borders becoming less than even side-to-side, etc.