Tiers are an unofficial method of classifying Pokémon within a particular competitive battling format based on their relative usage.
Individual sites, users, or organizations may publish tiers, but the most prominent tier lists are produced and published by Smogon, usually based on usage stats from unofficial battle simulators, currently Pokémon Showdown. Communities such as Pokémon Online and Pokémon Perfect have also published their own tier lists, though they are defunct as of 2023. Pokémon Online is of particular note for introducing usage-based tiering, the system most commonly used today. Tiers are defined separately for each generation, with their own rulesets, bans, and often even communities.
Each tier functions as its own metagame, where only Pokémon that aren't in a higher tier can be used. By categorizing Pokémon in this way, Pokémon that would normally be outclassed in higher-tier metagames can still be useful in lower-tier formats, much like weight classes in sports like boxing. This also allows for rules to be enacted that better accommodate the Pokémon in that metagame that would not be suitable for those in higher tiers. Thus, the purpose of tiers can be boiled down to ensuring that as many Pokémon are viable as possible.
In addition to usage-based metrics, some Pokémon are banned from a tier due to being considered too powerful for that tier's metagame. These can be for metagame health, being statistically overwhelming, or having strategies that players of the tier consider uncompetitive. In doing so, the tier can become more diverse, allowing for more Pokémon to be playable in the problem Pokémon's absence.
Tiers for the latest generation of games are typically updated more often than older generations, due to those usually being the most played formats and being usage-based. Usage-based tiering is conducted through battle simulators such as Pokémon Showdown collecting aggregate data on the Pokémon used in a given metagame and weighting it based on the player's Elo rating. The weighted usage percentage is used to prevent "noise" — such as outliers who use a Pokémon a considerable amount and invariably lose — from potentially resulting in unrepresentative data. Once the result is unveiled, Pokémon are then risen and dropped between tiers that they are perceived as being better suited for, with an objective usage percentage being employed. With this system, when a new generation of Pokémon begins, it can take multiple months for a new tier to become playable. Due to usage-based tiering relying on a considerable amount of players, once a generation has concluded, the data becomes extremely unreliable. Ergo, old generations of metagames are often locked once the next one begins, with the tier list considered complete and lower tiers being stabilised. However, retroactive bans may be enacted by those who continue to play the metagames, and Pokémon that fall out of favour may be marked as such on battle simulator platforms while still being kept in the tier for the sake of transparency.
In Generations I through III, usage-based tiering was not invented yet and they thus instead use viability-based tiering. With the disadvantages of usage-based tiering becoming impossible after a generation has concluded not being applicable, small communities of enthusiasts still tier these generations in small competitive communities, with websites like Pokémon Perfect notably being founded for this very purpose. Viability-based tiering is conducted through having tier lists for the metagame itself — dubbed "Viability Rankings" — produced through aggregate rankings either by qualified players or a general large-scale vote. Pokémon are sent up and down tiers based on their perceived viability in the current metagame, usually with a rank being used as the threshold, mirroring that of the given percentage in usage-based tiering.
If a Pokémon meets a certain threshold of usage within a particular tier, it is classified as being within that tier, and cannot be used in any lower tiers. A Pokémon's classified tier is only its lower limit; it can still be used in any higher tier. Some Pokémon may still be useful in higher tiers despite their classification but are simply not widely used, and vice versa; so while a strong guideline, usage does not necessarily equal viability. Additionally, some Pokémon — most famously Quagsire — may ebb in and out of metagames, thus travelling up and down tiers rapidly.
If a Pokémon has multiple forms that it cannot switch between within battle, those forms may have usage calculated separately from each other (eg. Giratina's Altered and Origin formes). However, if the form difference is purely or mostly aesthetic, those forms are often grouped together and tiered on their own (eg. Gastrodon).
If a Pokémon is banned from a tier, it is placed in a special BL (Borderline) group for that tier, meaning it cannot be used within the tier it's designated for, regardless of its usage. However, it can still be used in higher tiers. "BLs" are not considered to be officially playable tiers.
Because the metagame shifts over time, as well as Pokémon being banned and unbanned, Pokémon's usage within a given tier will vary over time. At certain points in time, the publisher of the tier system will update which Pokémon are in which tiers based on current usage or viability.
Different generations have different numbers of tiers. Newer generations have more Pokémon and more interest, so typically have more tiers as a result. Publishers of tier lists usually only create new tiers when there is enough interest in them to justify maintaining that new tier.
Tiers are normally ranked as follows.
- AG (Anything Goes, taken from the Stadium series of games)
- Ubers (German for "above competition", not normally tiered by usage)
- OU (OverUsed)
- UU (UnderUsed)
- RU (RarelyUsed)
- NU (NeverUsed)
- PU (expression of disgust)
- ZU (ZeroUsed, unofficial)
Some publishers of tier lists used the LU (Little Used) tier in place of RU or changed their definitions to be clearer (eg. 1U, 2U, etc). However, due to the prominence of Smogon's tier list, Smogon's terminology has dominated most tier lists.
Each tier functions as its own metagame, with its own banlist, normally referred to as the tier's name appended with BL (e.g. UU-BL). However, OU's banlist has its own unique name, Ubers. Playing without any restrictions (or only very specific rules such as the "Endless Battle Clause") is typically referred to as "Anything Goes," which has since been adopted as the banlist for Ubers.
Unlike other banlists, Ubers functions as a metagame itself. Due to its functioning as a metagame, Ubers has its own banlist dubbed "Anything Goes", for Pokémon that are considered too unhealthy even within its own metagame. This technically began in Generation IV with Arceus due to technical difficulties on Shoddy Battle, but officially beginning in Generation VI with Mega Rayquaza's official ban from Ubers. Unlike tiers, Ubers is not based on usage, so usage within Ubers does not result in a Pokémon ceasing to be in the OU tier, though tiering from Ubers has historically been proposed.
In addition to banning specific Pokémon, each format may also ban certain moves, Abilities, items, or even the usage of particular game mechanics (such as Dynamax or Terastal). They may also issue bans on certain combinations of these, such as a party not being allowed to have two named moves at the same time (particularly involving Baton Pass)) though this is considerably rarer and subject to extreme contextual circumstances.
Battle simulators typically offer tier-based formats by default, automatically enforcing the restrictions associated with them. To play with these formats in the games themselves, both players need to agree to the rules in advance, as in-game rule-creation options typically do not offer enough granular control over the rules to support these formats. Due to both the difficulty of enforcing these rules in-game and the availability of usage data, tier lists are usually based on usage within battle simulators.
Fans usually discuss tiers in the context of standard Single Battles. However, tier systems have also been published for other variants, such as Double Battles, Middle Cup, and Little Cup Single Battles. Tier systems for these format types function in the same way, but typically do not have as many tiers as standard Single Battles due to the lower popularity of these format types.
Similarly, some sites (such as Smogon) create their own custom Pokémon, and have format types that include those Pokémon with their own tier lists, in this case being called Create-A-Pokémon (CAP).
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