Episode Code Number

Most shows have titles for each episode, but there are also internal episode code numbers. These are codes given to each episode to help identify episodes. The code numbers usually match the broadcast order, but sometimes (usually due to Executive Meddling) the episodes are shuffled around.

There are three main ways to code episode:
 * Sequential Numbering: The first episode is given the code "#1" and the numbers increment from there. This is the standard way to number long-running programs that don't have episode titles, such as Game Shows.
 * Seasonal Numbering: The code is a number that is a bit like room numbers. The last two digits are the episode number and the first one or two digits are the season number. So "#421" would be the 21st episode of Season 4. This can also be written as "4x21", "4.21", "4-21", or "S04E21".
 * Weird Numbering: The episode codes here are a jumble of alphanumerics. Internally, this probably is useful for something, but only the die-hard fans will bother to learn the code. Casual fans will probably apply one of the above codes to the episodes.

Often, Pilot episodes aren't given a code. Some retroactively are labeled "Episode 0".

Anime

 * While not the official codes, Pokémon episodes are among the fandom generally assigned a two-letter prefix (EP, AG, DP, BW, or SS) followed by a three-digit episode number. EP038, for example, is the thirty-eighth episode of the original series (Electric Soldier Porygon, for those wondering). AG049 refers to the forty-ninth episode of the Advanced Generation series, and so on. "DP" refers to the Diamond & Pearl series, "BW" refers to the Black & White series, and "SS" refers to the 'side-story' episodes occasionally shown on Weekly Pokémon Broadcasting Station or Pokémon Sunday, dubbed under the Pokémon Chronicles banner or as individual specials. The movies are designated with an M followed by a two-digit number (e.g. M1 is... well, the first movie), and a small handful of episodes do not have codes as a result of being aired Out of Order.

Live Action Television

 * Shows like The Daily Show or The Colbert Report have sequential episode numbers, because episodes don't have titles, and seasons aren't as important to this kind of show. Due to the news-esque nature of the shows, it's more convenient to just refer to episodes by their airdates.
 * Doctor Who used an increasing number of letters from Seasons 1-26 (A-Z, then AA/BB/etc. and later 4C, 7D, then Seasonal Numbering with the revival. A list is here. Note the three codes for The Trial of a Time Lord—that 14-part story is effectively made up of four separate chunks (The Mysterious Planet, Mindwarp, Terror of the Vervoids, and The Ultimate Foe), with the last two produced together as a single block and sharing a code (7C). Also notice that the codes for some Season 30 (Series 4) episodes don't match the broadcast order—this is not a case of Executive Meddling, but of the show's creators altering the plot as the season was being filmed; they kept the original codes so as to avoid confusion.
 * According to the BBC, the current run of Doctor Who is not the same as the old run, thus the Tenth Doctor ended his tenure in "Doctor Who (2005) Series 4".
 * Big Finish gives the production code "8A" to the 1996 film, and their Who audio dramas with Paul McGann continue the numbering as you'd expect (8B, 8C...).
 * Lexx had episode numbers that are one season off. The pilot films are numbered 1.0, 2.0, 3.0, and 4.0. The first "full season" (Mantrid) was numbered onscreen as 2.xx, the second one (Fire and Water) is numbered 3.xx, and the third one (Little Blue Planet) has 4.xx numbers.
 * Lost numbered its episodes hour by hour, such that a two-hour season finale is considered internally to be two episodes. As a result, "The Variable" was promoted as the 100th episode when in fact it is actually the 96th episode and the 100th hour.
 * On the DVD commentaries, the pilot is not counted toward the total; "Tabula Rasa" is referred to as the first episode. By this logic, "The Variable" should have been #95.
 * Not a unique phenomenon, as many programmes have an opener or finale which is two hours. In the case of Heroes, the opening credits even named the two episodes separately and billed the credits as such. "Directed by: Richard Richard ("Cock"), Edward Hitler ("Bull")".
 * For the current run of The Price Is Right, the daytime run originally used a "D" designation corresponding to the week number and day of that week—for example, #1412D was the Tuesday show of the 141st week (aired May 13, 1975). Once the show reached Week #1000 in May 1996, they switched the "D" to "K" and went from #9995D to #0011K, skipping a week.
 * This was done mainly to distinguish the daytime show from the nighttime version, which started at the same time and used codes ending in "N".
 * The first five daytime episodes used a second production number that prefaced the normal "D" -- "#0101-X", with "X" referring to the taping order from 1-5. The first week's tapings were #0011D, #0013D, #0014D, #0012D, #0015D. There is no way to tell how this system would have continued (the next taping, #0022D, did not have an alternate code), but a logical conclusion may be that "#0101-X" could have also been written as "S01W01E0X".
 * There were also several times an entire episode had to be scrapped and replaced (including #0101-2, above). One of these resulted in the episode number (1513K, which would have aired September 27, 2000) being changed to 1513X.
 * Nighttime specials also used different codes. The first six specials, aired in 1986, used "P"; the 25th-Anniversary Special used "S", while the 30th-Anniversary Special used "LV". All specials after that have used "SP".
 * Mythbusters, due to the number of special episodes that may or may not be part of a series, has seen fans using several different and contradictory schemes. Of course, Mythbusters is unusually well-suited for The One With...... designations, so many fans simply name the episode by its primary myth.
 * Barry & Enright Productions identified each episode of their shows in syndication in a season by the letter and number. For example: A-001 = First episode of season 1.

Video Games

 * Telltale Games' The Adventures of Sam & Max: Freelance Police series uses the "Seasonal Numbering" method.

Web Original

 * RWBY is somewhat idiosyncratic in that it doesn't have "seasons", it has "volumes"; aside from that, episode coding is handled with a fairly straightforward "V00E00" system.

Western Animation

 * Both The Simpsons and Futurama have funky episode codes, like 7F19 or 2ACV06.
 * Family Guy and many other FOX shows also have strange episode codes, like 2ACX08. This seems to be a standard format for FOX shows, especially in the earlier years of 20th Century Fox Television.
 * In the days of single letters, the letter referred to the production—F was The Simpsons from season 2 onwards, X was The X-Files, and Space: Above and Beyond had S.
 * In the modern numbering system, the three letter code refers to the show, and the starting number (or letter in The Simpsons case) indicates the production season.
 * The season 4 premiere episode of Teen Titans is actually titled "Episode 257-494", its production number. As a gag, Control Freak's prisoner number is shown in a mugshot as 257-325, the production number of his first appearance.