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I see terms like OU, UU, etc. but I don't know what they mean.

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There are many half-baked answers to this question on the internet, so I will try to make this answer thorough. I use a deliberately vague definition at the start and slowly build up from there, so you don’t have to untangle everything as you go.

What are tiers?

Tiers are a ranking system for the effectiveness of each Pokémon in competitive play. Generally, a Pokémon in a higher tier is viewed by competitive players to be better than a Pokémon in a lower tier.

Tiers in Pokémon (as we know them) were created by a community called Smogon in 2004, and are still run on the Smogon Forums. All the official tiers for single battles are listed below, in order of strength (i.e. Ubers is highest and PU is lowest).

Each tier also doubles as a playable battle format. A battle format (or simply ‘format’) is a ruleset that specifies what Pokémon are allowed, what strategies can be used, which clauses are in effect, etc. during a battle. You may already know the ‘Battle Stadium Singles’ (BSS) format on your Sword/Shield cartridge. Official VGC tournaments also have their own formats.

When playing a battle in a particular tier, you are only allowed to use Pokémon that are rated within that tier, or rated below that tier. For example, if I’m playing Gen 8 OU, then I can use Dragapult because it is rated OU. I can also use Conkeldurr because it is rated UU in Gen 8. However, I cannot use Palkia, because Palkia is rated Ubers in Gen 8. To see a list of Pokémon in each tier, use Smogon’s strategy dex.

Smogon tiers/formats and their rules are not enforced by the cartridge games themselves, because tiers are fan-made and moderated by fans. To play ranked matches for Smogon tiers/formats, use the popular simulator Pokémon Showdown! (or just ‘Showdown’).

There are also tiers for doubles, called Doubles Ubers, DOU and DUU. These are less popular than the singles tiers because the doubles community generally revolves around VGC.

How do tiers work?

Tiers are fundamentally usage based. This means: if a particular Pokémon is used often enough in a certain tier, then it will become a member of that tier officially. Of course, the highest tiers are the hardest to join, because the best Pokémon are in them. Tiers are not decided by the opinion of a group of people – they’re decided by the habits of real players.

To decide if a Pokémon gets “enough usage” to be part of a particular tier, Smogon uses the rule:

A Pokémon truly belongs to its tier if a typical competitive player is more than 50% likely to encounter that Pokémon at least once in a given day of playing (15 battles).

Mathematically, this means that a Pokémon must maintain at least ~4.52% of total weighted usage in players’ teams for a particular tier to remain in that tier (or get promoted to it). Official usage stats are published by Smogon, so you can see which Pokémon are close to being promoted or demoted. These are taken from ladder matches on Showdown.

Every month, on the month, Smogon administrators officially announce the usage-based changes for the tiers (also called the ‘tier shift’, not to be confused with the battle format Tier Shift). Here is an example of what this looks like. If a Pokémon has failed to make the cutoff for its current tier (~4.52% weighted usage) then it will drop to the tier below. If a Pokémon has achieved the ~4.52% cutoff in a tier higher than its current one, then it will be promoted to that tier.

(ASIDE: A Pokémon in PU can suddenly jump up to OU if it gets enough usage in OU, i.e. skipping multiple tiers. Massive changes like this have happened before – new releases and developments in the metagame can make Pokémon with a niche skillset suddenly more valuable. In this sense, tiers cannot be thought of as a perfect ranking system – ultimately, they are at the mercy of living, breathing metagames that might need certain traits more than others. If OU players really need a specially-defensive Grass-type for some reason, then specially-defensive Grass-types could get more usage in OU compared to UU.)

Though tiers are primarily usage-based, manual bans can also occur. Each official tier has a council on Smogon that, if it deems necessary, can host a suspect test for a particular Pokémon, item, etc. During a suspect test, a new ladder on Showdown is created with the proposed change (e.g. [x] item is banned, [y] Pokémon is unbanned), and players who get a high enough Elo rating on that ladder will be able to vote on whether the proposed change goes through.

This creates a small complexity: what happens to a Pokémon that is banned from UU, but isn’t officially OU by usage? This is where the ‘borderline’ (BL) tiers come in. A Pokémon banned from UU but not used enough in OU to get promoted is given the tier UUBL (and likewise for RUBL, NUBL, etc.). For example: at the time of writing this, Gengar is banned from Gen 8 UU, but its recent usage in OU is only 2.14%, so it is designated UUBL.

BL tiers are not playable tiers. A Pokémon in UUBL is OU for all intents and purposes – BL tiers are just there to distinct between Pokémon which truly belong in their tier, and those which were ousted from a lower tier. Note, there is no BL tier between Ubers and OU; by tradition, Ubers literally is OU’s banlist, so all Pokémon under ~4.52% usage in Ubers but banned from OU are still formally Ubers.

What about AG, LC, etc.?

AG and LC are both playable battle formats, but they are not part of the tier ecosystem described above. Usage in AG and LC has no bearing on the official tiers (and they have different philosophies as far as their rules are concerned), so they are dealt with separately.

It is still possible to “rank” Pokémon as AG or LC if they don’t belong to an official tier (and Smogon’s strategy dex does this), but be wary that this is not a real assignment. Zacian-Crowned (which is banned from Ubers in Gen 8 and often labelled “AG” as a result) is formally untiered, and so are all of the Pokémon in LC (or otherwise) that don’t get enough usage in any official tier.

Note, this means each Pokémon does not necessarily have a tier. If a Pokémon does not get enough usage in any official tier to become part of an official tier, then it will just stay in untiered purgatory. Such untiered Pokémon can be used in any official tier. (They are sometimes given “ZU”, which is an unofficial tier below PU.)

What should I do?

If you are new to competitive Pokémon, you should get comfortable with tiers, and consider building a team for a tier that you like the look of. For example, if you like Gyarados, then you might want to play UU, because Gyarados is rated UU and is a popular choice in that metagame. Read strategies on Smogon for the tier you’ve chosen and maybe look up teambuilding guides to help you make a structure.

Remember to pick your battle format before teambuilding, so you can respond to common threats in the metagame while you’re teambuilding. Many new players build a team, don’t know where to go, and end up queueing AG ("Anything Goes") – don’t do that!

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Why did you answer this question and not https://pokemondb.net/pokebase/27282/what-is-a-tier ?
I don't mean to nitpick or anything, but Scizor is OU
@sumwun a. the old answer on this thread is better compared to the answer on the other one (which is outright incorrect on a couple things), b. this thread climbed SEO ranks and the other didn't.
@dankmuffin lol my bad.
Oml nice answer Fizz
I think these links might be helpful, in case people read this answer and still want to know more.
Tiering FAQ: https://www.smogon.com/forums/threads/tiering-faq.3644714/
Tiering policy: https://www.smogon.com/forums/threads/tiering-policy-framework.3628026/
Not really important, but LC, AG and four other tiers were UMs (Unofficial Metagames) in Gen 7, but now they're OMs (Other Metagames), as UMs were scratched off.
5 votes

Tiers are fan made tiers. By tier I guess you could call them categories for Pokemon.
They are used for competitive battling to split each Pokemon up based on how they fair in each tier. The higher tiers consist of powerful legendary Pokemon or Pokemon with abilities that make them just to good for Ou. Ou is the second highest tier standing for Overused and is basically all your Ubers rejects like Thundurus-T and then allot of standard Pokemon that are to good for Uu or just have abilities that could possibly make them overpowered in lower tiers.

The whole concept of tiers are so you don't face a full team of Legendary Pokemon every match when your using your Magikarp. You battle and build your teams around the Pokemon in the tier but you can use Pokemon below the tier in your team. The most used tiers are Smogons who are a major Pokemon community possibly with 20x the amount of members this site has. With their many members they put together tiers based on the Pokemons typings, abilities, threats, movepool and stats. Typing is a big deal it is what keeps some Pokemon like Charizard in Nu because when switching Stealth Rocks takes away 50% of its health.

Here are smogons tiers enjoy

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