Stage legality

Stage legality concerns the playable status of all the stages in competitive play. Each stage is classified into one of three possible categories: neutral, counterpick, or banned.

Neutral stages, also sometimes referred to as "starters", are a collection of diverse, yet balanced, stages that are always available to use during a set. The most important function of a strong neutral list is to give as many characters as possible the chance to fight on even ground during the first game of a set. Due to the way stage striking works, there must always be an odd number of neutral stages, with five being the norm.

Counterpick stages are more situational stages that are only available after the first game of a set. These stages are overall less balanced than neutrals, giving a noticeable advantage or disadvantage to certain characters that would cause issues if the stages were included on the neutral list. Despite this, these stages are considered to be acceptable for competitive play. Counterpick stages tend to be picked to counter certain characters, matchups, or playstyles, hence the name.

Banned stages are not allowed in competitive play. These stages possess one of a number of possible flaws that disrupt the nature of competitive play, often by either encouraging degenerate tactics or by introducing randomness that promotes luck over skill. Common characteristics of a banned stage include the following:
 * Providing an extreme advantage or disadvantage to a particular set of characters. For instance, large stages with high platforms give a disproportionately large advantage to characters with good movement options by allowing them to circle camp against less mobile characters.
 * Possessing overpowered stage hazards that can KO characters or otherwise get in the way of the fight. While Super Smash Flash 2 has a hazards switch, some stages, such as Bowser's Castle, have static hazards that remain active even when hazards are disabled.
 * Possessing any of the following physical characteristics:
 * A "pit of death". This gives a large advantage to zoning characters by limiting the approach options of an opponent, while giving a heavy disadvantage to because he doesn't have enough room to recover with PK Thunder should he fall into the pit.
 * A permanent walk-off edge. This allows degenerate tactics like easy zero-to-deaths while de-emphasizing or nullifying the offstage game entirely. The "stand near the blast zone and back throw the opponent when they get close" strategy is the most prominent example of this degeneracy.
 * A "cave of life", an area that possesses walls or a ceiling that makes it very difficult to KO a character. Examples include the infamous "Hyrule Fight Club" on Temple or the titular cave in Emerald Cave.
 * A permanent wall, if it's tall enough. This provides a strong camping position while granting characters access to broken combos that wouldn't normally be possible on another stage.
 * Having an exploitable game-breaking glitch, such as getting stuck in the terrain in an unreachable location.
 * Causing framerate issues due to overloading Flash with too many detailed elements. In Beta, fully customizable graphics settings were implemented, making this a much rarer occurrence that usually only depends on the PC running the game.
 * Being too similar to another legal stage. This is mostly an issue in tournaments with conservative stagelists with few strikes during the counterpicking phase, where players may be forced to waste an extra strike to get rid of two near-identical stages.
 * Dramatically changing the way the game is played or the strategies required to win, such as in moving stages like Butter Building or Urban Champion.

The Gentleman's Agreement is usually allowed as a way for players to use banned stages as long as both players agree to it, though sometimes it's restricted to only the legal stages.

Banned stages in Super Smash Flash 2
Many rulesets have existed for Super Smash Flash 2. As such, there exists no popular consensus for the legality of all stages. For the purpose of this list, some extra categories will be included, indicating that a given stage may be neutral, counterpick, or banned under certain rulesets.