Twelve-Episode Anime

Series running about half the length of the standard 24-26 (depending on holidays) episode season. These shows are Short Runners, but for these, that was never a bad thing. Scheduled for a 3-month airing slot if aired weekly.

This format has become popular (see 12 and 13) in recent years as a compromise to the sometimes sporadic nature of OVAs and the longer, sometimes filler-ridden, standard season. Being a smaller time investment and financial risk, it is much easier to determine if a show will be successful. For this reason, many Western live-action shows have adopted this schedule, at least at first.

Shows that are successful often get another (12-episode) season, essentially achieving a "normal" length. When released on DVD, a bonus episode is often included, making the set a Baker's Dozen.

Most of these series tend to air at Otaku O'Clock. See also British Brevity.

Series with 12 episodes

 * .hack//Legend of the Twilight Bracelet is 12 episodes long, but was also an adaptation of a monthly serial.
 * Akane-iro Ni Somaru Saka -- twelve episodes plus a Fan Service OAV.
 * Asobi Ni Iku Yo plus an OAV
 * Ben To
 * Black Blood Brothers
 * Black Lagoon had two seasons of this kind, the second called "The Second Barrage".
 * Blood-C
 * Boku wa Tomodachi ga Sukunai
 * Boogiepop Phantom
 * Chaos;Head
 * Cowboy Bebop in its initial airing was 12 stand-alone episodes and a filler "recap" episode, due to it being too violent for its time slot. The rest of the series was made at the time, but they decided to leave out the more violent episodes for the first run.
 * Cubex Cursedx Curious
 * Dancougar Nova: The last half especially feels very rushed and underdeveloped.
 * Dance in The Vampire Bund
 * Darker Than Black's second season fits, but not the first season and its 24 episodes.
 * The sequel to Deep Love, Deep Love ~Host~.
 * Denpa Onna to Seishun Otoko
 * Devil May Cry the Animated Series
 * Dragon Crisis
 * Ef a Tale of Memories and its sequel Ef a Tale of Melodies
 * El Hazard: The Alternative World
 * Fortune Arterial: Akai Yakusoku
 * Yoake Mae Yori Ruri Iro Na ~Crescent Love~ is an earlier example of one of August's Visual Novels adapted into a Twelve-Episode Anime.
 * Freezing plus two OVA episodes.
 * Futakoi Alternative and its predecessor Futakoi.
 * Ga Rei Zero
 * Grenadier
 * The "La Verite" remake of Hanaukyo Maid Tai has 12 episodes. So far, no sign of a second season.
 * He Is My Master (No sequel series yet)
 * All three seasons of Hidamari Sketch: the first has 12, the second has 13, the third 12 again. However, between the DVD-only bonus episodes, and some TV specials broadcast some months after the main series run, the totals are 14, 16 and 14 respectively.
 * Highschool of the Dead plus an OVA episode.
 * Hitohira
 * Ichigo Mashimaro. Two OVA series, one with three episodes, the other with two, were made after the TV series, so there are now 17 episodes of Ichigo Mashimaro.
 * Innocent Venus: unfortunately because of some bad pacing the middle was painfully slow but the later episodes felt too rushed.
 * Iron Man
 * Kaiba
 * Kamen no Maid Guy has one 12-episode season so far, but there's rumors of another one following soon.
 * Kamisama no Memochou
 * Kannazuki no Miko, and quite a bit of other works originally created by Kaishaku, for example:
 * Magical Nyan Nyan Taruto.
 * Both UFO Ultramaiden Valkyrie TV series.
 * Kyoshiro to Towa no Sora (a.k.a. Shattered Angels).
 * Kashimashi Girl Meets Girl
 * Koihime Musou. All 3 seasons have 12 episodes.
 * Kore wa Zombie desu ka? plus one OVA episode.
 * Kure-nai
 * Kurozuka
 * Legend of Himiko
 * Loveless. The manga has 9 volumes, still ongoing.
 * Mahoromatic got 12; its sequel series (subtitled Something More Beautiful) got 14.
 * Mahou Tsukai Ni Taisetsu Na Koto
 * Mahou Tsukai ni Taisetsu na Koto ~Natsu no Sora~
 * Makai Senki Disgaea, the anime based loosely on the game.
 * Manabi Straight plus an OVA, which takes place between the 6th and 7th episodes, to make this series 13 episodes when counted.
 * Maria Holic
 * Maria-sama ga Miteru and its second season Maria-sama ga Miteru Haru.
 * MM!
 * Mononoke
 * Which is itself a spinoff of twelve-ep show Ayakashi: Samurai Horror Tales (and more specifically, the four-episode story "Bakeneko" at the end).
 * Najica Blitz Tactics
 * Nanatsuiro Drops
 * Night Walker: A unique case, as the first four episodes were originally released as an OVA series. The following eight episodes were then aired as a television series.
 * Ninin ga Shinobuden, a.k.a (Ninja Nonsense and 2x2=Shinobuden)
 * Please Teacher and Onegai Twins are twelve episode animes, with one-episode OVAs each.
 * Ore no Imouto ga Konna Ni Kawaii Wake ga Nai has twelve broadcast episodes plus four OVA episodes.
 * Potemayo
 * Princess Princess ran for 12 episodes following three of the five volumes of manga. No word yet on whether the manga sequel, Princess Princess Plus will be getting an anime adaptation.
 * Prism Ark
 * Puella Magi Madoka Magica
 * Queens Blade
 * Rocket Girls
 * Rozen Maiden and its sequel series, Rozen Maiden Traumend.
 * Sacred Seven
 * All three of Sayonara Zetsubou Sensei's seasons are 12-13 episodes long.
 * School Days, along with two OVAs
 * Shingetsutan Tsukihime
 * Sola
 * So Ra No wo To
 * Taishou Yakyuu Musume
 * Umi Monogatari
 * Uta Kata, which also had a postlude OVA.
 * Venus Versus Virus had 12 episodes...but ended on a Cliff Hanger.
 * Wandering Son. It's a Twelve-Episode Anime, with only eleven episodes being broadcasted on television and one unreleased one being added onto the Blu-Ray.
 * Wolverine
 * both seasons of The World God Only Knows are both 12 episodes long each.
 * Yami to Boushi To Hon no Tabibito
 * Yosuga no Sora
 * Xenosaga: The Animation

Series with 11 episodes

 * Aoi Hana
 * Bunny Drop
 * C the Money And Soul of Possibility 
 * Eden of the East: 11 episodes and two movies.
 * Fractale
 * Hakaba Kitaro
 * Jyu Oh Sei
 * The Tatami Galaxy
 * Kuragehime, although
 * Moyashimon
 * ~Tokyo Magnitude 8.0~
 * All of the above were aired on ~noitaminA~, which does this often. (If you know what the programming block does during the gaps between series, please replace this parenthetical note with that information.)
 * Kämpfer is an odd case; it's 12 episodes, but it's basically an 11 episode series with episode 12 being a fanservicey, non-continuity bonus episode that's aired after the ending such as it is.
 * Zombie Loan plus two DVD episodes.

Series with 13 episodes

 * AIR TV: though it had 13 episodes, the last was a Recap Episode and generally discounted.
 * The Recap Episode wasn't even included in the FUNimation English DVD release.
 * There are also two OVA episodes called AIR in Summer.
 * Akane-iro Ni Somaru Saka -- thirteen episodes plus a Fan Service OAV.
 * Angel Beats, which was supposed to have 26 episodes and was unexpectedly cut down to 13. A bonus OVA was released later, and the disk set came with an unaired BLAM Episode.
 * Baccano got 13, although episodes 14-16 were released later as bonus material.
 * The Big O, though it was later renewed for a second season.
 * The Big O is notable for being one of the first Twelve Episode Anime to get a second season, picking up where the first one left off, after a lengthy break, something which was almost unheard of in the anime industry at the time.
 * Birdy the Mighty Decode got 13 episodes; its second season got 12.
 * Canaan
 * The Monkey Punch anime Cinderella Boy. The series ended  and originally suggested an upcoming new season. Unfortunately, the high cost caused the series to be put to a premature end.
 * The original series Deep Love was thirteen episodes, while the spin-off Deep Love ~Host~ ran for twelve.
 * Digimon Adventure started as this, as a spinoff to the original Digimon Adventure movie (the first part of Digimon: The Movie as shown in North America), but it did well enough to warrant the rest of the 54 total episodes. Considering how self-contained the File Island arc is, it shows.
 * Divergence Eve has two thirteen-episode seasons.
 * ~Dual! Parallel Trouble Adventure~ has thirteen episodes, plus a special.
 * Elfen Lied is a Thirteen Episode Anime, plus the 14th DVD-only episode.
 * Figure 17 has 13 episodes, but is somewhat unique in that each episode is 45 minutes long, and one episode was shown each month during its first airing. As such, the anime takes significantly more time exploring and developing its characters, has a gentler pace, and animation and art that's practically OVA quality.
 * It was later rebroadcast as a standard 26 episode show, with each episode cut in half.
 * Flag
 * Gravitation, 13, cut so heavily it would need a sequel series to bridge the gap to the manga's sequel series...
 * Gunslinger Girl, with 13 episodes and a 13-episode sequel, as well as a 2-episode OVA.
 * Haibane Renmei, written by Lain's character designer Yoshitoshi ABe, is also a 13-episode series.
 * Hellsing got 13 episodes. It needed more to tell the story it was trying to.
 * That's only because the author pulled the plug on it because he apparently didn't like where it was going. All funding was cut around episode 10-11, and they had just enough to wrap up the series in the next two-three episodes.
 * Hyakko' has 13 episodes and a short 12-minute-long OVA.
 * The first Kanon TV adaptation is 13 episodes long plus a special. The KyoAni remake was a full season with 24 episodes.
 * Ikki Tousen's first season. Its second season had only 12.
 * Jubei-chan: Secret of the Lovely Eyepatch and the second season Counter Attack of the Siberia Yagyu.
 * Kannagi
 * Kazemakase Tsukikage Ran
 * Key the Metal Idol -- though it came out as a total of 15 episodes: 13 regular, plus two "movies".
 * Android Kikaider: The Animation has 13 episodes once "recap episode" #8 is added in. However, this episode was not shown during Kikaider's Adult Swim run, nor was it put onto DVD.
 * The TV series is followed up by a 4-episode OVA, loosely based on the Kikaider 01 tokusatsu series.
 * Kino no Tabi (Kino's Journey in the English DVD release) is another 13-episode series.
 * Kokoro Library
 * Legends Of The Dark Kings (aka Ten no Haoh), a 13-episode anime adaptation of a manga spinoff of Fist of the North Star that centers around Raoh.
 * Magical Girl Lyrical Nanoha and its sequel series Magical Girl Lyrical Nanoha As compacted 26 episodes of plot into 13 high-pressure episodes; however, Magical Girl Lyrical Nanoha StrikerS was the standard length (and pace) of a 26 episode series.
 * Mayoi Neko Overrun! has twelve episodes plus a metafictonal thirteenth episode of the characters doing commentary on the series.
 * Mezzo DSA
 * Midori no Hibi got 13 episodes (which is probably why two major characters from the manga, Lucy and Nao, were left out).
 * Naru Taru got 13 episodes. 13 episodes that literally left out half the plot of the manga it's based on.
 * Neo Angelique: The Anime of the Game Spin-Off is two sets of 13 episodes.
 * Now and Then Here And There
 * Panty and Stocking With Garterbelt, though Gainax apparently has a second season planned.
 * Paranoia Agent
 * Prétear
 * Saishuu Heiki Kanojo (Saikano / She, the Ultimate Weapon)
 * Sasameki Koto
 * Seikimatsu Occult Gakuin, or simply Occult Academy
 * Serial Experiments Lain, true to its indie nature, is a thirteen-episode series.
 * Shinrei Tantei Yakumo
 * The Silver Brumby's only season had 13 episodes.
 * Soukou no Strain
 * Spice and Wolf had 13 episodes, with the 7th episode being DVD-only. The second season had 12 episodes, with an epilogue to the last season as an episode 0 on DVD.
 * Starship Operators
 * Stratos 4 has 13 episodes in its first season. It's second season has 6 episodes plus a 2-episode OVA, and there is another intermediary two-part OVA.
 * To Heart
 * To Heart 2 as well, however, it has a three part OVA, and a two part OVA spinoff To Heart 2:Another Days, which is loosely based on the Gaiden Game spinoff of the same title.
 * Tokko
 * Tona Gura has 13 episodes with a semi-firm conclusion. That said, the manga is still ongoing, and whether or not this conclusion is used remains up in the air. Up until 2009, manga chapters tended to be fully fleshed, expanded versions of the anime episodes.
 * Windy Tales
 * Working
 * Vandread first aired with 13 episodes, but managed to get a second 13 episodes and resolve the story. There's rumors of a third season, but aren't there always?
 * Yami no Matsuei
 * Zero no Tsukaima has 13 episodes, although its two sequel seasons have 12 each.

Series with 14 episodes

 * The initial run of The Melancholy Of Haruhi Suzumiya got 14. The second season also has 14 episodes (a total run of 28, including the first season).
 * One of the first season's episodes is the Non-Indicative First Episode, which is a "movie" created by the SOS club rather than a normal episode.
 * Also consider that 8 of 14 episodes in season two are generally the same.

Odd cases

 * Aria The ANIMATION and The ORIGINATION (Season One and Season Three) are both 13 Episodes in length whereas ARIA The NATURAL (Season Two) is 26 episodes long.
 * Asagiri no Miko has 26 episodes, but each is under a quarter of an hour long, half the "standard" episode length.
 * The same goes for the Galaxy Angel anime.
 * And Steel Angel Kurumi
 * Same goes for Cromartie High School, though they generally broadcast them in pairs.
 * As well as Risky Safety, but it has 24 episodes.
 * W~Wish and Final Approach each have 13 twelve-minute episodes, aired side-by-side during the "Princess Hour."
 * Ayakashi: Samurai Horror Tales is an unusual case; it's a 12-episode series and was aired as such on Japanese television, but is actually an anthology consisting of three separate 4-episode animes by completely different production groups. (The final series in the anthology, "Bakeneko", ended up with its own spinoff--Mononoke, also a 12-ep series.)
 * Blue Literature follows the same format, with five arcs adapted from Japanese literature.
 * The anime series of Black Rock Shooter is eight episodes long, and was aired in Noitamin A. If you count the OVA (which was released two years earlier and is in a separate continuity), then it had nine episodes in total.
 * As mentioned in the main article, many live-action Western shows are adopting this, such as NBC's Chuck. During the 2007-2008 season, however, a lot of this was the unintentional result of that season's Writer's Strike.
 * Fate Zero's first season has 13 episodes, but it's effectively 14 episodes, because the first episode is 45 minutes long. This was apparently done to get the humongous amount of prologue exposition (for a show which is itself a prologue for an entire franchise) out of the way in the first episode, so the second season will probably be a normal 13 episode season.
 * Full Metal Panic's two sequel seasons, Fumoffu, and The Second Raid had 11 and 13 episodes broadcast, respectively. Fumoffu was originally scheduled for 12 episodes, but one episode was pulled from broadcast due to its kidnapping plot having parallels in Real Life at the time. The Second Raid had a separate 30 minute OAV produced afterwards.
 * Kamichu! had 12 to start, with four DVD-only episodes interspersed between the broadcast episodes. (A ploy to get people to buy the DVDs, as the DVD-only episodes are not strictly filler.)
 * K-On's first two arcs are only twelve episodes, but the show ended with a special episode outside of the second arc. Then it sky-rocketed in fame and fortune, and the second season got 26 episodes.
 * Shiki has 22 episodes plus an OVA episode 20.5.
 * The fourth season of Slayers ending up being split into two 13-episode seasons instead of one 26-episode season. Of course, Revolution leaves a lot of plot threads for Evolution-R to pick up, but season five did get a new OP and ED...