Here are a few of my recent ACEO's. They are 2 layers, one pen and ink, the other is watercolor. The pen and ink is cut out with an x-acto knife and mounted on top of the watercolor. The edges of both are blackened with ink as well.
ACEO stands for Art Card Editions and Originals. The size of an ACEO is always 2.5" by 3.5". They are created in a variety of medias and are highly collectible works of art.