Definitions for "Double Mat"
Layering two colored paper mats on top of each other, allowing the bottom mat (mat closest to the image) to have a 1/4 -1/2 inch reveal.
two mats, one over layed upon the other, with the bottom mat cut 1/4th of an inch (on all 4 sides) smaller than the actual print and the top mat 3/8th of an inch (on all 4 sides) larger than the bottom mat and usually of a different color for visual purposes.
A technique where the artwork is matted using two separate mat boards, one on top of the other. The amount of the bottom mat you see is determined by the offset.