The Kar Keepers are designed to protect and display Hot Wheels, for the collector to keep his or her Hot Wheels safe for collecting. Also known as a clamshell case, or protective clear plastic sleeve. They are designed to fit all Hot Wheels basic cars and Hot Wheels Classics. In 2007 Mattel came out with another line of Kar Keepers for those collectors collecting Ultra Hots, which have a larger card than your basic Hot Wheels card. These larger Kar Keepers also fit the larger cards used for the Dragstrip Demons series.