Set Bonus

A subtrope of Gotta Catch Them All referring to certain items bestowing benefits only when the player has a full or partial matching set of them. These sets are usually predetermined and have an official or unofficial collective name or a common theme to indicate that they should be used together.

The resulting bonuses are often better or more difficult to obtain than those on regular items without extensive upgrades. While individual pieces of a set may be cheap (the most common ones can border on Vendor Trash at times), getting them all at once jacks up the price to a good deal more than the total cost of the pieces. An emerging trend is to have a set offer some of its bonuses for just two pieces and add on the rest as more pieces are worn, allowing Munchkins to mix and match the bonuses by wearing parts of multiple sets.

Players can usually cobble together a combination of regular and/or unique items with overall stats as good as or better than a matched set of their level, but this requires more effort and resources (not to mention luck) over a longer period of time. In short, the Set Bonus mechanic offers a wholly in-game shortcut to power for people who can afford the larger down-payment. Also worth noting is that it (usually) looks better than Rainbow Pimp Gear.

Most of the time there is no In Universe explanation offered for why these items have to be together to produce the extra effects. It could be said that the enchantments need to 'overlap' or that the pieces when connected form the circuit of a system that then produces the effects. Most games with this mechanic will also have each item's description "kindly" inform the player of the potential benefits, without any proper explanation of just how the character could know about them. Maybe they were written somewhere on the items themselves.

This trope is absurdly prevalent (though by no means universal) in MMORPGs but it turns up in other genres as well. It is often used to encourage trading among players but it has the side effect of making the more obsessed sink extra time and money into a game.

Anime & Manga

 * In The Slayers, Lina has four magical amplifiers (that she bought off of Xellos) that work better in tandem then separately.

Board Games

 * In Monopoly, the player is only allowed to build houses and hotels on properties where they own the entire color block.
 * Risk provides additional troop bonuses when you occupy an entire continent.
 * As well as when you trade in three cards with matching symbols.

Tabletop Games

 * The most ubiquitous of this trope in Magic the Gathering is the "Urzatron": Urza's Mine, Urza's Tower, and Urza's Power Plant. They give one mana each normally, but if you control all three, you get seven mana from them.
 * Speaking of Magic, Visions had Feral Shadow, Breathstealer, and Urborg Panther. Sacrifice all three of them to bring Spirit of the Night into play directly from your deck.
 * Having Helm, Shield, and Sword of Kaldra all in play (each a strong card in their own right) and you can put a 4/4 legendary token creature (aptly named Kaldra) into play and attach all three of them to it for a much lower cost than attaching individually.
 * In Yu-Gi-Oh, the five Exodia cards are worthless on their own. Get all five in your hand, and you automatically win the game.
 * There are also dozens of archetypes that entire decks can be built around (Six Samurai, Laval, Ice Barrier, Evol, Archfiends...)

Literature

 * The eponymous artifacts in Harry Potter were believed to make the owner of all three Master of Death, granting immortality.

Live Action TV

 * In one episode of White Collar, a thief was collecting a set of jade elephants that became much more valuable when all of them were sold together. Neal tried to lure her in by offering to team up so they could sell the full set and split the profits.
 * Kamen Rider OOO's medals come in sets of three, and using a set of medals of the same type gives him increased powers at the cost of leaving him exhausted when he powers down.

Video Games

 * Diablo 2 had these.
 * Rings sets in zOMG!.
 * Armor in Dragon Age gives additional bonuses if it forms a matching set.
 * Certain weapon combos do the same, such as Maric's Sword and Cailan's Shield. Most fall into that sword-and-board type, but the expansion Awakening offers up a sword and dagger set bonus for dual wielders.
 * Certain rings also provide additional benefits when paired together.
 * Certain armor in Torchlight does this. It actually tells you a) that it's in a set, b) the name of the set, and c) what bonuses you'll get for getting more pieces.
 * Expansion packs to Heroes of Might and Magic III introduced these, as certain sets of items when equipped result in absurdly powerful bonuses. In fact, two of the campaigns in Shadow of Death revolve around gathering those... for the Big Bad. Another campaign revolves around splitting one. The campaign that ties all of the stories together revolves around the heroes' efforts to reunite the divided set to counter the ones owned by the Big Bad.
 * World of Warcraft has an armor set for each class for each tier for both PVP and PVE. That's a lot of sets.
 * There are also sets you can craft, and sets that can get dropped. There are also sets of 2 items, 3 items, 4 items, etc... including one set that's nothing but two one-handed weapons.
 * Many, many sets of equipment in Dissidia Final Fantasy come with bonuses when they are all equipped.
 * Having three matching viviosaurs of a set on your team in Fossil Fighters is necessary for using their team attacks. What makes a set varies, but it can be things like similar styles, diets, or elements.
 * In Animal Crossing, you get bonus points for your HRA score by collecting pieces of furniture sets, themes, and series.
 * City of Heroes has enhancement sets.
 * Tactics Ogre Also used the set equipment variant, usually giving the character a lesser weight penalty.
 * In Pokemon HeartGold and SoulSilver, depending on their version, the player gets an item that lets them capture either Kyogre or Groudon. Regardless of which one they caught, however, Oak's reaction is to state that he thinks of the pair as a set, and wonders if you can capture the other one. However, you can only capture one of the pair in each version -- and the next event won't be triggered by any normal Groudon/Kyogre -- you specifically need one who was lured in and caught by the exclusive HG/SS item.
 * Most of the Gatito pins in The World Ends With You only work as a set.
 * The "Black Planet" pins work on their own, but all of them increase in power considerably when used together.
 * Ratchet and Clank: Size Matters has armor pieces that can be discovered while playing through the levels. Collect and equip a matching set confers an additional bonus, such as adding acid or fire to your melee attacks.
 * Chrono Cross - collecting a special set of equipment for Pierre turns him from Joke Character to Lethal Joke Character.
 * Dragon Quest VII uses this during the Style Contests in Litorud. Matching certain weapons, armor and accessories together (like Bunny Ears, Bunny Suit, Bunny Tail and a Silver Tray) gives you bonus points for coordination.
 * Runescape has few sets with set bonuses.
 * Final Fantasy XI has two different kinds of set bonuses. The first and by far the most common type requires that all pieces of a particular set be equipped in order to get the effect, which for the most part isn't worth it, though the Usukane Haramaki set is fantastically powerful for Monks and Puppetmasters, and the full Perle Hauberk set is good for Dragoons and Dark Knights. The second type, used only in Campaign armour, allows for a partial bonus if two or more pieces of the set are equipped. Unfortunately, Campaign armour is so underpowered that even this doesn't make it useful. Rainbow Pimp Gear still rules the day for the majority of jobs.
 * The Lord of the Rings Online has sets of armor, sets of jewlery, and sets that are a mix of both. However, some class-specific armor sets have a fairly useless bonus for all six pieces, suchs as a 10% chance of instantly cooling down an ability that already has a short cooldown timer.
 * Dynasty Warriors Gundam gives stat bonuses for equipping parts made by the same inventor, and special skills for equipping multiples of the same part type.
 * You get a one-time bonus in Rise of Nations's Conquer-The-Entire-World campaign for controlling a complete continent.
 * Outfits in Kingdom of Loathing.
 * In Billy vs. SNAKEMAN, some teams of allies have extra bonuses on top of individual ally bonuses.
 * Wearing all three pieces of a cosplay set during BillyCon gives you that set's "combo", a fourth cosplay piece for all intents and purposes.
 * Monster Hunter has every set of armor only provide bonus skills and resistances when the full 5-piece set is worn--or at least combined with equipment that gives similar skills.
 * In Adventure Quest, mastercraft items have a set bonus--for example, the Golden set having you heal half the damage you deal on a special attack.
 * Fly FF had the green items (which were not green, but had their name in green). Usually people would only wear green items because their bonuses and the amount of time you could use them was far greater than the normal items. Of course they were also really expensive.
 * Team Fortress 2 uses these, with one for every class but The Engineer. They are quite blatantly Revenue Enhancing Devices, being difficult to craft in game and being offered in a bundle for a discount. However, the second wave of sets don't require the themed hat to get the bonus, so they are more feasible to collect without spending real money.
 * This is parodied by the "Dual-Purpose Fruit" set, of which one component is actually a long existing item, as the effect is "Reduces the chance of hunger by 13%" 0 is in fact equal to 87% of 0.
 * Mega Man X's armor system in X8 allows him to interchange parts between the two armors, Icarus and Hermes. However, if he instead equips a complete set, a special ability of that armor will be available for use: Icarus comes with the series' trademark Giga Attack, while Hermes has the X-Drive, increasing the capabilities of the Hermes armor further.
 * The Junk Armor in Mega Man Zero 4 comes in three pieces: head, body and legs. Only when Zero equips all three parts should the armor's ability be in effect: doubling his attack power while halving his defense.
 * Dwemer armor is supposed to work like this if the wearer has a complete set, lorewise.
 * In Terraria, Combining three armor items of the same type (which cover the legs, chest and head) gives the player an additional bonus. Hardmode armors have bonuses which depend on the type of headgear you have.
 * Marvel Ultimate Alliance gave bonuses depending on who was in your party, usually if they were part of a team like The Avengers or the Fantastic Four.