At the bottom of this graphic is a horizontal axis labeled "Backup level" at the left end and "Up to nine levels" at the right end. The following sequence of numbers appears below this axis, starting at its left end: 0, 1, 2, 3, 2, 3. Vertical lines extend from each of these numbers to the top of the graphic.

Five horizontal arrows appear in the graphic, all pointing left and joining the following points: from 1 to 0, from the first 2 to 1, from the first 3 to 2, from the second 2 to 1, and from the second 3 to the second 2.