Artist trading cards, or ATCs, began in Europe. Originally ATCs were snippets of canvas, pieces of sketchbook pages, or bits of watercolor or drawing paper with contact information and sometimes a bit of painting or drawing. They were a quick, informal way for artists to network.

Artist trading card, a tiny abstract painting by Kathleen Gresham (2 1/2 by 3 1/2 inches)
Gradually ATCs become more and more standardized in size and shape and started to be made and exchanged for their own sake. People discovered that making and trading artist trading cards is quick, easy, and fun.

Eventually a standard size was agreed upon: 2 1/2 by 3 1/2 inches. That particular size allows artists to protect their cards with the inexpensive, archival plastic sleeves used to protect other trading cards like baseball cards and game cards, such as those for Magic: The Gathering. It also allows us to store and display our card collections in three-ring binders in the vinyl pocket pages that hold nine cards each.

A 3D assemblage ATC by Kathleen Gresham, an origami crane, folded from hand-painted paper and mounted on layers of glittery painted cardstock.

Aside from the size, the only hard-and-fast rule about artist trading cards is that they may be either freely given as gifts or exchanged for other ATCs, but they must not be sold.

If you like the size and want to create miniature artworks in that size to sell, the proper term for those is ACEO or miniature.

To view a collection of videos showing artist trading cards and how to make them, click on following link: ATC Demos