In CD lingo, panel refers to each individual page or square area of your cover. For example a single panel cover is a single piece of paper that is not folded, and is printed on only one side of the sheet. It is blank on the inside panel. A two panel cover is that same sheet of paper with printing on both sides of the paper so that when you open the case you see one page of additional information that you can't see from the outside. A 4 panel cover is a folded sheet of paper making a booklet. When the paper is closed you see panel number 1 which is the front cover of the package. When you open the booklet you see panels number 2 and 3, or the two inside panels. When you close the booklet you see panel number 4. Don't confuse this with the tray card, which is a separate sheet of paper that goes underneath the tray that holds the CD. It is the back of the package. We also offer 6 panel inserts which are also known as two fold inserts, and 8 panel inserts which is (2) 4 panel inserts saddle stapled together.