A printing plate is the actual thin metal plate used to print cards during the manufacturing process. There are typically four plates per card — one each for cyan, magenta, yellow, and black (CMYK) — the four colors that combine to create the full-color image on your card.
Each printing plate is a genuine 1-of-1 collectible, since there's only one plate per color per card. They're raw, industrial artifacts from the production process, and they offer a fascinating behind-the-scenes look at how cards are made.
Printing plates appeal to collectors who value uniqueness and manufacturing history. They're not the prettiest cards in a collection, but they're always conversation starters.