Plot Parallel

Essentially, the A-plot is repeated in miniature in the B-plot. By looking at the results of one, the main character or audience gains a greater understanding of the other. This is a fairly common variation on Two Lines, No Waiting, as it gives the story layers and depth while remaining concise. It allows you to more fully explore each story -- they prop one another up. Sometimes takes the form of a Show Within a Show.

For example, suppose the A-plot has the heroine trying to get a pair of rare birds to mate. The subplot is the heroine is in denial about being in love with her best friend. By getting the birds together, she realizes she is in love with her friend. Another example: A lawyer defends a client who is accused of incompetence because of his age. The B-plot shows the lawyer worried that he's too old for the job. In defending his client, the lawyer realizes he's not ready to retire yet.

Often results in a Double Aesop, but not always; not all examples have characters learning a lesson. For example, the main plot of King Lear (Lear banishing faithful Cordelia and being betrayed by his other daughters) is mirrored in the Gloucester subplot, where Gloucester disowns faithful Edgar and is betrayed by his illegitimate son Edmund. The two stories are obvious parallels, but nobody learns anything from them (except the audience, who learns to be horribly depressed).

Sometimes this is the plot purpose of the Beta Couple.

Anime and Manga

 * Exaggerated in Puella Magi Madoka Magica. For the first two episodes, you'll be wondering where Deconstruction comes into play. And then every other episode has some horrifying revelation. Madoka, however, has not become a Magical Girl, and is essentially The Load. Episode 10, however, is something entirely similar:
 * Pluto:

Comics

 * Watchmen is chock full of parallelism, but probably the most obvious example is the fake comic "Tales of the Black Freighter;" the sailor's desperate and violent struggle to save his family from the Black Freighter (which ends in him killing an innocent man and joining the Freighter's crew) is an allusion to.
 * Also by Alan Moore, the plot of The Killing Joke is paralleled by the joke at the end. How the parallel works can be interpreted many ways.

Film

 * Sita Sings the Blues: The author of the movie learns by reading the Ramayana and eventually making it into this movie.
 * Signs, where the "miraculous" defeat of the aliens helps the protagonist to overcome his crisis of faith.
 * In The Fall, the fairytale that Roy tells Alexandria have many parallels with past and present events in the lives of the two main characters.
 * In Harm's Way is an A, B and C. It follows three couples falling in love: Admiral and Nurse; Ensign and Ensign, and Lieutenant and wife, during World War 2.

Literature

 * Look To Windward has a lot of parallelism. Every major event and character is reflected by another. The war that motivates Quinlan? There's another one. Quinlan's desire? . Masaq's decision to retire? Two people do that for similar reasons. Kabe's discovery that he's been absorbed by The Culture when he meant to study it?
 * In the novel The English Patient, the love story of Kip and Hana provides several parallels to that of Almasy and Katherine, and points up some of the novel's major themes, particularly that of man-made divisions (geography, racial discrimination) being the source of many of the world's troubles. The movie, by diminishing the Kip/Hana story and altering its resolution, doesn't really embody this trope the way the novel does.
 * In Rodrigo y el libro sin final (Rodrigo and the unfinished book), the titular character, a nine-year-old boy, helps a novelist suffering from writer's block to find an ending for a book he borrowed from the library. In the process, the writer discovers that some events in the book can be put in parallel with his own life: he and the pirate whose adventures he narrated left their girlfriends to live their dreams; both of them are now old men; both of them feel guilty when the past reappears and have to make a decision. The ending
 * In The Hitchhiker's Guide to the Galaxy, the book begins with the main character waking up to see that his house is being demolished without proper notice. The demolisher then states that he did have proper notice, but it is revealed that there was no way he could have actually known about, or seen the notice. Then, spaceships appear all over the earth and it is revealed that the earth is scheduled for demolition and that they were given "proper notice..." Luckily, the main character survives.

Live Action TV

 * JAG: In "Into the Breech," the A-plot is some navy cadets are holding a mock trial for a sailor who was badly hazed and involved in a Love Triangle with one of his bullies; the B-plot is pretty much the same, except it's between the cadets holding the mock trial.
 * Wonderfalls did this pretty much every episode. The bird example comes from "Safety Canary": A pair of rare birds refuse to mate (A-plot), and Jaye is having love problems with Eric (B-plot). After spending the entire episode using the birds to avoid interacting with Eric, Jaye realizes she can't give him up just because she's scared of commitment.
 * Lost: This happens in almost every episode, with the flashbacks/sideways paralleling what is happening on the island. For instance, in one season six episode focusing on Ben, in the flashsideways Ben has to make a choice between power and Alex. In the main story, he is forced to deal with the consequences of having already made that choice.
 * Huge: The troubled romance/sexual tension between Amber and one of the counselors is paralleled by the Twilight expy Phantasma, which the girls of the camp have been fangirling over. The forbidden love comparison becomes even more apparent when scenes of Amber and the counselor are shown between scenes of the Phantasma movie.
 * Scrubs, along with Double Aesop, virtually Once an Episode, and lampshaded about Once a Season.
 * 7th Heaven was another show that did this constantly, to drive home An Aesop even more anviliciously.
 * The West Wing also did it frequently. At the end of one episode where the A-plot is Sam finding out shocking, identity-threatening truths about people he cares about, he tells Donna, "It's just that there are certain things you're sure of, like longitude and latitude." Funny he should put it like that considering one of the B-plots was C.J. and Josh learning about the inaccuracy of common map projections and how the world literally isn't what they thought it was. (To be fair, Donna does Lampshade this.)
 * Wishbone plots parallel the classic stories the title character reads.
 * Pushing Daisies (created by the same person who created Wonderfalls) did this a fair bit as well.
 * Alexis Castle and her problems normally parallel some aspect of the case her father and Beckett are working, or some aspect of their budding relationship. Leads to many a Eureka Moment.
 * ICarly and its five episode Sam/Freddie romance arc ended with one of these. In the A-Plot, Sam and Freddie were trying to become closer. In the B-Plot, Spencer and his older babysitter (he was 10, she was 15) entered a relationship that got creepy as she started acting like his babysitter again and bossing him around. Carly ends the B-Plot by telling Spencer and the babysitter that their relationship is creepy and weird, and that they were forcing a romantic connection out of their babysitter/child relationship. Sam and Freddie overhear this, and both realize that even though Carly wasn't talking about them, her words matched their dysfunctional relationship. They discuss it, then break up.
 * Fringe starts doing this very effectively in the second and third seasons where the case-of-the-week symbolically parallels the developments in the Myth Arc.

Theatre

 * This is long established in literary criticism, and most of Shakespeare's plays follow it in one form or another.
 * The aforementioned King Lear
 * The Tempest, the story of Caliban becoming a servant of two sailors mirrors the relationship between Prospero and Ariel.
 * A Midsummer Nights Dream has three plots running side by side with play-within-a-play, the relationship between Titania, Oberon and Bottom as well as the central love quadrangle.
 * The framing device of The Taming of the Shrew mentions a man who doesn't want to go home because he can't control his wife. The main plot of the play...
 * Christopher Marlowe's Doctor Faustus has the A-plot of Faustus selling his soul for power. His servant Wagner uses Faustus's books to learn how to do the same, and then his newly-found servant Robin does the same thing. Taken Up to Eleven when Robin takes on his friend Dick as his own servant...
 * Oklahoma's subplot is a comic version of the A-story's Love Triangle, with the Beta Couple having the same issues as the main couple.
 * Leoncavallo's opera Pagliacci: Canio, a Commedia dell'arte actor who plays a cuckolded fool on stage, finds himself cuckolded -- but he refuses to be the fool.

Video Games

 * In Deus Ex Invisible War, there's a minor B-Plot about two rival coffee companies, and their desire to wipe each other out. It turns out . Later on, the player discovers that two factions in the A-Plot are doing the same thing.

Western Animation

 * Avatar: The Last Airbender
 * In "The Great Divide," the B-plot is about Sokka and Katara, who are bickering about one being messy while the other is neat. The A-plot is that two tribes are bickering, because one is messy and the other is neat. This leads to a subversion of the Double Aesop, because although Sokka and Katara make up easily, the tribes don't.
 * In "Bitter Work", Aang has a hard time learning Earthbending in the main plot. Meanwhile, in the B-plot, his rival Zuko is learning a difficult Firebending technique, to control lightining. The parallel shows how the two characters deal with frustration, showing their contrasting personalities all while advancing the plot.
 * Batman: The Animated Series: "Mean Seasons" is about a former model who was fired because she was too old and avenges herself by kidnapping her former employers; in the B-plot, Bruce is upset at losing an employee because he has hit the mandatory retirement age (Bruce is also feeling a bit sluggish, and even begins checking himself for gray hairs). In the end, Bruce does away with mandatory retirement.