1. The ATCs MUST BE the specific dimension of 3.5" x 2.5" (64 x 89mm).
2. They are NOT to be sold, only traded/exchanged/swapped.
3. On the back of each ATC there needs to be this information: the artist's name, contact information, title of the ATC and number if needed.
A single, unique ATC is called an original, it is one-of-a-kind; two or several identical ATCs are called editions and are numbered: 1/5, 2/5, 3/5 and so on; several ATCs with one theme but each is slightly different are called series.
Card stock or a sturdy paper such as watercolor or acrylic paper is used for the card itself. Recycled cardboard or even a deck of cards can be used. There are online templates for cutting several ATCs out of one 8.5" x 11" sheet of card stock:
ATC Template PDF
ATC Cutting Template PDF
As to theme, techniques, design, anything is permissible and strictly up to the individual artist. Acrylic paint, watercolor, oil paint, ink, felt tip pens, gel pens, rubber stamping, cut up a cards or paper for collage, charcoal, pastels, pencil, 3D, fabric, image transfers, paintings, drawings, can all be used to make the art design. The ATCs can be color or black/white or a combination.
Once the ATC is finished, pictures or computer scans can be taken of the original so that prints can be made to use in other personal art work, cards, or notebooks.
I enjoy history so I wanted to find out about the history of ATCs. According to Wikipedia, the ATCs
are individual art miniatures which pass hand to hand. Their current popularity among artists and hobbyists was sparked by M. Vänçi Stirnemann, who began trading sessions in Zurich, Switzerland, in 1997, and popularized ATCs via the internet.
There's more history at the original ATC website and information about how to make them, where to trade, and a gallery to view ATCs made by various artists.