An "ATC" is an Artist Trading Card.  Each card is 2.5" X 3.5'. That is basically the only rule. They can be made of mixed media, fabric, mini watercolors, prints of original work and even embellished with jewels. You get the idea! Originally, these cards were traded among artists. A more recent development is the "ACEO" which stands for Art Card Edition Original. These are the same cards, but sold by artists. They are great fun for everyone to collect! I have started selling them in my etsy store. Last time I checked, there were over 2,000 available for purchase! They are the perfect little gift for a friend and the prices vary, but they are certainly less expensive than a larger piece of work... and a great opportunity to get an affordable original from your favorite artist. I am also selling mini mats in my store so that you can put your ACEO in a ready made standard 5 x 7 frame. Anyway, take a look and let me know what you think. I welcome any ideas for new cards, too.