You can think of a mortise mask as the "field" surrounding your image. Some people think of it as the "negative mask". For the ATC below, the mortise mask was large enough to cover the whole ATC, but with a bird-shaped hole cut out of it. Sorry I didn't take photos of the process, but hopefully it makes sense.
|Perhaps a sage outline around the bird would've been better?|
I have to admit when I think about the time it took to achieve this effect, I marvel at how simple it would be to do via PhotoShop! Virtual "cutting" is so much quicker, lol. On the bright side, I liked the design enough to stamp a few all-occasion greeting cards in addition to this ATC.