Front 13 Back 9

Friday nights - that's when Fox advertises its 12-episode DVD sets. -Tim Minear

Network TV shows usually have about 22 episodes per season. But networks rarely have enough confidence in a new show to order all 22 at once. Typically the first 13 episodes will be ordered, followed by the remaining 9 if ratings are high enough. These sets of episodes are referred to as the front 13 and back 9, with the latter term being more commonly used. So far so dull, but the 13/9 structure does tend to bleed through into the actual show in interesting ways. The front 13 are often produced before any episodes are aired and before there can be any public feedback. If the creators are doubtful about the back 9 being ordered, the 13th episode often contains some kind of resolution so the audience isn't left hanging. Short Runners often have 12 or 13 episodes produced, even if not all of them were aired.

Examples:
 * 24 has Bauer's wife and daughter rescued in the 13th episode, providing some resolution if the back 9 was not ordered.
 * Dollhouse had a DVD-only 13th episode due to confusion between the network and studio about the number of episodes in the order. It was set in a postapocalyptic future to avoid disrupting the show's story arcs with an episode that wasn't aired.
 * Glee had a crucial competition win, resolution for several story arcs, and even a traditional kiss in the final scene in its 13th episode. All of the front 13 were produced before airing and have fewer songs, less use of Sue Sylvester, and no theme episodes.
 * Buffy the Vampire Slayer is a subtle example. The front 13 were produced before airing, so footage from all of them was available when editing the pilot. So in the scene where Buffy thrashes around in bed having prophetic dreams, she can only predict the first 13 episodes...
 * Firefly is essentially the front 13 (as well as the pilot), and nothing else.