The 2008 Topps Heritage High Numbers set includes an additional 35 black back semi-short print cards. Each black back card has a corresponding green back card that is exactly the same, except for the color of ink used on the back of the card. Since the total production of each number is the same for each base card these 35 cards are approximately twice as hard to pull as a standard common card. For example, let's say card # 501 John Danks has 1 million cards printed by Topps (1 million is just an easy number for the example, not the actual number). Since all of John Danks cards are green back cards, there are 1 million green back John Danks cards in circulation. If there are 1 million John Danks cards printed by Topps, then there will also be 1 million # 514 Brandon Backe cards printed by Topps, however since there is a green and a black back version of Backe's cards, there will be approximately 500,000 green backs and 500,000 black backs. Still 1 million total, but each version is twice as rare as the Danks card.