ATCs stands for Artist Trading Cards.
They are miniature works of art, 2.5 per 3.5 inches, supposed to be traded between artists as a small demonstration of the creators artwork.
When created for selling purposes, they are normally numbered and called ACEO’s that stands for Art Card Editions and Originals.
On a Double Buddy Jam, the first person sends 2 similar ATCs to a second person. The receiver works on top of those starters, keeps one and returns the other to the sender.
A Triple Buddy Jam is rather similar, yet involves 3 artists.
On the ATCs back, the receiver may sign and write the date when he received and the date of the art creation.