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Thank you to Gamestop's Game FAQ's for this info.

If you're new to competitive battling, or just confused by all the crazy rules people seem to be making up lately, you might not know what people refer to as the standard rules. Here they are.

No Ubers
This means that the following Pokemon are banned:
- Arceus
- Darkrai
- Deoxys (all forms)
- Dialga
- Giratina
- Groudon
- Ho-oh
- Kyogre
- Latias (both with and without soul dew)
- Latios (both with and without soul dew)
- Lugia
- Manaphy
- Mew
- Mewtwo
- Palkia
- Rayquaza
- Wobbuffet

Sleep Clause
You may only put one Pokemon to sleep at a time. Once that Pokemon wakes up, you may put it or another Pokemon to sleep again. But again, only one Pokemon on their team can be asleep at one time. Pokemon asleep due to the effects of Rest or Effect Spore do NOT count towards that limit.

Species Clause
Each Pokemon must be of a different species. Different Pokedex number means it's a different species.

Evasion Clause and OHKO Ban
Sometimes grouped together as "No DT/OHKO". The following moves are banned:
- Double Team
- Minimize
- Fissure
- Guillotine
- Sheer Cold
- Horn Drill

Everything else, including Sand Attack, the item Bright Powder and the Sand Veil ability, are legal.

Non-Standard Rules that are sometimes used
The following rules aren't included as standard. However, they are quite popular among many users and make for a different style of play. If they are stated alongside the standard rules, remember to abide by them.

Item Clause
By far the most common non-standard rule, item clause states that each Pokemon on the team must be holding a different item from one another. So no double leftovers or focus sash. Items that do the same thing but aren't the same, such as leftovers and black sludge, are allowed.

No "Hax" Items
The following items are banned:
- Brightpowder
- Quick Claw
- Focus Band
- King's Rock

Self-KO Clause
This mostly only applies to tournaments. It's used to break ties. If both players are down to their last Pokemon and one uses a move that kills itself, such as Explosion, Self-Destruct, Destiny Bond, or Memento, the trainer who used that move is considered the loser in the match.

Freeze Clause
Only used in a controlled environment such as Pokemon Battle Revolution, Shoddy Battle, or the upcoming Competitor. Only one Pokemon may be frozen at the same time as another. Once one unfreezes, that Pokemon or another may be put back to sleep.
Firestorm88
Posted 10/2/2007 9:58:23 PM
message detail The UU Metagame
Above, the "No Ubers" rule is in place. Some people like to go a bit further than that and play in the "Underused" metagame. Most battles are in the standard OU (overused) environment. The UU environment just expands the list and bans Pokemon who fall in the Overused and Borderline tier as well:

Overused:
- Aerodactyl
- Azelf
- Blissey
- Breloom
- Bronzong
- Celebi
- Cresselia
- Donphan
- Dragonite
- Dugtrio
- Dusknoir
- Electivire
- Forretress
- Garchomp
- Gengar
- Gliscor
- Gyarados
- Heatran
- Heracross
- Hippowdon
- Infernape
- Jirachi
- Lucario
- Machamp
- Magnezone
- Metagross
- Porygon-Z
- Rhyperior
- Salamence
- Skarmory
- Slowbro
- Starmie
- Suicune
- Swampert
- Tangrowth
- Togekiss
- Tyranitar
- Vaporeon
-Weavile
- Weezing
- Yanmega
- Zapdos

Borderline:
- Abomasnow
- Alakazam
- Ambipom
- Arcanine
- Articuno
- Blaziken
- Charizard
- Claydol
- Cloyster
- Drapion
- Empoleon
- Entei
- Espeon
- Exeggutor
- Floatzel
- Flygon
- Gallade
- Gardevoir
- Hariyama
- Honchkrow
- Houndoom
- Jolteon
- Jynx
- Kingdra
- Leafeon
- Lickilicky
- Ludicolo
- Magmortar
- Marowak
- Medicham
- Mesprit
- Milotic
- Miltank
- Mismagius
- Moltres
- Ninjask
- Porygon2
- Raikou
- Ramparados
- Regice
- Regigigas
- Regirock
- Registeel
- Roserade
- Sceptile
- Scizor
- Slaking
- Shaymin
- Shedinja
- Smeargle
- Snorlax
- Spiritomb
- Staraptor
- Steelix
- Tauros
- Torterra
- Umbreon
- Uxie
- Venusaur
- Zangoose

The above list is tentative and may change in the near future. It is the Smogon tier list as that is the accepted standard.

Remember than when playing Pokemon you are agreeing to your opponents rules before the battle begins. Even if it's something crazy like "No Legendaries." If you don't like the rulest, don't play them.