In general, all stages fall into one of three categories:
Neutral stages may be chosen in any game of a set, including the first game. These are also referred to as starter stages (not to be confused with the term for stages that do not need to be unlocked to be used in-game).
Counterpick stages may be chosen only after the first game of a set, such as games 2 and 3 of a best-of-3 set. Stages in this category tend to give a slight, though not overwhelming, advantage for certain characters over others, such that it would not be fair to include them in the neutral list. These stages may often (though not always) be chosen for counterpicking purposes, hence the name of the category.
Banned stages cannot be chosen at all. Stages can be banned for several reasons, common reasons including but not limited to:
The term legal stage can also be used to describe any neutral or counterpick stage, or equivalently, any non-banned stage.
In general, due to the wide variety between stages and stage types, almost all of the stages within any given game are banned and very few are balanced enough to be legal for professional competitive play.
Despite the banned stages list, most tournaments feature some variation of the "gentleman rule", a rule that allows players to choose any stage they want, including banned stages, if all players in the match agree to it. This is usually the only way for banned stages to see serious tournament play. However, even with this rule in effect, it rarely sees use to play on banned stages outside a much higher-skilled player letting an opposing more casual or very young player choose any stage they want, as most competitive players never want to willingly play on the banned stages for the same reasons the ruleset had them banned. Even with the rule in effect, TOs may still ban some stages from being played on via the gentleman rule, if the stage has a propensity to delay the tournament by causing much longer matches (such as Temple, New Pork City, and The Great Cave Offensive).
Major differences in communities
There are many community preferences in terms of stage choices, with there being no agreed-upon standard across all regions on what exactly constitutes a "proper stage", so a stage's legality can be dependent on the region the tournament is in; this is especially true in Brawl and Smash 4, due to a lack of an officially standardized stage list. Pokémon Stadium 2 in Brawl, for instance, was generally banned in the Tristate Area, while tournaments in Texas usually allowed the stage to be a legal counterpick choice.
The commonality of characters can also play a role in what stages are legal or banned. Again in Brawl, the rising prominence of Meta Knight in the metagame led to stage lists in most regions becoming more restrictive. In the Tristate region, Meta Knight was not banned from tournaments, and as a result, the Tristate tournament scene banned or otherwise restricted stages such as Delfino Plaza, Halberd, and Rainbow Cruise due to Meta Knight's overwhelming power on such stages. In some regions, however, Meta Knight was not as widely used as the Tristate Area; as such, some of the aforementioned stages have differing legalities, with Halberd sometimes being a starter stage and Frigate Orpheon going from a banned stage to a counterpick. WHOBO 4, the first American MK-banned major, featured an expansive stagelist that included Delfino Plaza, Halberd, Rainbow Cruise, and Brinstar as counterpicks, even though most of these stages were banned in MK-legal tournaments at the time.
The following is Smash 64's banlist for stages in tournaments. Until mid-2015, four stages — Dream Land, Hyrule Castle, Congo Jungle, and Peach's Castle — were legal. Currently, Dream Land is the only stage that can be used in tournaments. However, with tournaments switching from the vanilla version of the game to the 19XX mod, in some tournaments like GENESIS 5 or GENESIS 6, the stages Battlefield and Final Destination, which weren't available without hacking, have seen some tournament presence in doubles.
Despite an official ban being placed on Hyrule Castle, the stage is still a rather common pick by players who often invoke it via the gentleman's clause.
In the current Smash Back Room rule set, all stages are listed definitively as a starter, counterpick, or banned stage, as follows:
In the past, the following stages have been legal counterpicks, but are now banned:
The following stages have never been legal in competitive Melee history:
The following stages are legal in the PMBR 2018 Full Recommended Ruleset 
Brawl has a highly fragmented and argumentative community when it comes to which stages should be legal. As a result, the stage legality list is significantly messier than the previous two games, with most stages being disputed in one way or another. Additionally, a stage's legal status is heavily influenced by Meta Knight's legality, as he tends to gain even more drastic advantages on many fringe stages. Thus tournaments that ban Meta Knight tend to allow more stages than tournaments that keep him legal.
This table shows the status of each stage using a four-tier system.
Additionally, the community at large has never considered the sample stages in the Stage Builder as official stages, and as a result, they are all considered banned. Despite this, none of the three stages would be theoretically legal in any ruleset: Hole has no grabbable ledges from the outside and contains a powerful camping location in the lower-left spikes; Bath frequently lacks the lower blast line and has walls that block the horizontal blast lines and allow for infinites; and Maze's irregular design promotes camping, circle-camping, and stalling, and the stage's architecture creates caves of life. Arguably, a large number of setups also often lack the stages, as they interfere with the loading of mods via the Smash Stack.
Stage legality in Super Smash Bros. for Nintendo 3DS
Since the release of its home console counterpart, Super Smash Bros. for Nintendo 3DS has had minimal tournament presence. The following stage list is used by Anther's Ladder for 3DS netplay and most online 3DS tournaments:
Stage legality in Super Smash Bros. for Wii U
Like Brawl, there is no uniform stagelist for Super Smash Bros. for Wii U, with some differences and disagreements on stage categories between areas. The table below generally uses the most commonly accepted legality for each stage.
Stage legality in Super Smash Bros. Ultimate
The list below shows the tournament legality for all stages in Ultimate. With the option to turn off stage hazards (which also turns off transformations) stages like Pokémon Stadium 2, Kalos Pokémon League and many more stages are potentially viable tournament stages. Note that this list assumes that hazards are turned off. For other stages that are not viable, here are their reasons:
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