A 9-pocket page, also called a 9-card page or a 9-card sleeve, is a 3x3 plastic page that holds 3 cards per row and per column. Other variations of this type of card pocket page exists. Additionally, some collectors prefer not to put more than one card in a pocket, as opposed to putting all duplicates in the same pocket (called a "double up"), or putting cards in backwards to be displayed on the reverse side of the page.2
Kaufeld, John; Smith, Jeremy (2006). Trading Card Games For Dummies. John Wiley & Sons. ISBN 0470044071. 0470044071 ↩
Miller, John Jackson (2003), Scrye Collectible Card Game Checklist & Price Guide, Second Edition, pp. 46–47. ↩