Artist trading cards
Artist trading cards (or ATCs) are miniature works of art about the same size as modern trading cards baseball cards, or 2 by 1⁄23 inches (64 mm × 89 mm), 1⁄2 small enough to fit inside standard card-collector pockets, sleeves or sheets. The ATC movement developed out of the mail art movement and has its origins in Switzerland. Cards are produced in various media, including dry media (pencils, pens, markers, etc.), wet media (watercolor, acrylic paints, etc.), paper media (in the form of collage, papercuts, found objects, etc.) or even metals or fiber. The cards are usually traded or exchanged. When sold, they are usually referred to as art card editions and originals (ACEOs).
M. Vänçi Stirnemann is credited in many circles with popularizing the modern artist trading card in 1996, holding trading sessions in Zurich, Switzerland. This resurgence of interest in the cards spawned the ACEO (Art Cards, Editions and Originals) movement.
ACEO: Art Cards, Editions and Originals 
An offshoot of artist trading cards are Art Cards, Editions and Originals (ACEOs), which originated when some artists began to create cards to sell, in addition to trading among themselves. Many ACEOs are sold on internet auction sites, such as eBay. As the term suggests, ACEOs may be small original works of art, or editions of small prints.
Michael Leavitt's Art Cards, hand-painted small portraits of artists, replicated traditional baseball trading cards in style and format. Subjects were drawn from various genres, and included Vincent Van Gogh, Bob Ross, Frida Kahlo, Jackson Pollock, and Michael Jackson. The limited editions were hand-packed in wrappers with bubble gum. Leavitt said, "It's a way I created to compare people and what they do across all kinds of different areas."
As for an earlier example, in 1991, Cleury Champion first sold his cards through the catalog of his avant-garde magazine eXpErImEnTaL (bAsEmEnT). Champion's most recognized cards were playing cards with experimental poems of John M. Bennett rubber stamped on the face.
Letterbox Trading Cards 
- Tracy Roos in Terry Taylor, Altered Art: Techniques for Creating Altered Books, Boxes, Cards & More, Lark Books, 2004, p103. ISBN 1-57990-550-1
- Opie O'Brien, Metal craft discovery workshop: create unique jewelry, art dolls, collage art, keepsakes and more!, North Light Books, 2005, p100. ISBN 1-58180-646-9
- Trading cards offer glimpse into Edmonton art scene Edmonton Journal Friday, July 27, 2007
- Lois Huey-Heck, Jim Kalnin, The Spirituality of Art, Wood Lake Publishing, 2006, p138. ISBN 1-896836-78-X
- Artists' trading cards put collectible creations in the palm of your hand Honolulu Star Bulletin Sunday, May 6, 2007
- Nishimoto, G: "Living Artist Are Just as Good as Dead Ones", Vapors Magazine, Fall 2005.
- Albert, J: "Watch Out for Darth Bush", South Seattle Star, June 25, 2003.
- "Stranger Suggests", The Stranger, December 28, 2006.
- Herber, K: "Art Cookin' in the Kitchen", Madrona News, March 2003.
- John M. Bennett Publications Collection, 1940-1995: Guide and Inventory, SPEC.CMS.107 - folder # 2677 and # 2943 from Ohio State University Rare Books and Manuscripts Collection.
- "Help: LTC". Retrieved 13 January 2012.