For the most part, it’s tradition. While there are indeed general league rules about gym types (mostly to avoid having multiple gyms of the same type in the same league), most gym leaders choose their types or themes, based on preference, family obligations, or other personal factors.

The inclination to do so has a lot to do with how the original leagues were established. Back in the feudal days, warlords of Kanto and Johto represented themselves with an element and a crest. In order to settle minor conflicts, clans sometimes sent representatives to battle high-ranking officials of each other’s armies using pokémon of their clans’ respective types. This way, disputes can be settled with very little bloodshed. These battles became the foundation of gym matches, crests became badges, and once the League was established and feudalism fell, anyone can become a gym leader (although some leaders, like Falkner, are indeed descendants of those old warlords).

As for the Elite Four and champion … similar principle, but they often represent the emperor and the imperial guard, whose battles were more for recreation than conflict resolution.

Excellent question!

The rules generally depend on the gym. They all need to be approved by the Association, of course, but whether the challenger is allowed up to six pokémon (i.e., League limits) or only as many as the gym leader will use (leader limits) is entirely up to the gym leader’s preference. Some gym leaders work with types or within arenas that are difficult to tackle with just a handful of pokémon; others believe that it’s a far better challenge to place additional limits on the challenger.

The same thing can be said for time and switch limits, actually. If you listen closely to a referee at the start of a match, they’ll usually announce whether or not you or the gym leader can switch pokémon and whether or not you’ll battle under a specific time limit. (If it’s timed, then the goal is to knock out as many of your opponent’s pokémon as possible within that time frame while avoiding losing many of yours. Whichever side has more pokémon standing or pokémon in better condition wins.)

Generally, the most common rules are as follows:

• Leader limit
• Skill matched (Meaning, the gym leader is using a team that matches your experience level as a trainer. When you walk into a gym and ask for a battle, you’re usually also asked for the number of badges you already have so the gym leader can put together a team that won’t immediately murder yours.)
• No time limit
• The challenger is free to switch pokémon; the gym leader cannot.
• The gym leader may only use up to two items, and neither of these can be revive or any derivative thereof. The challenger may use any item they wish.

Technically, battles within a gym must also take place within a sanctioned battlefield, of a certain length and width determined by the League and Association. If a battle takes place on such a field, then there are usually rules about TKOs, or instances in which a pokémon is thrown out of bounds. This is especially true for battles that take place in stadium arenas or arenas with spectator seats, as these rules confine the battle to a certain area and prevent bystander injury. As such, boundary rules are actually mandatory … if a gym battle is taking place inside a gym. (In truth, a gym battle can take place just about anywhere so long as a gym leader is present and states clearly that it’s a gym battle. Also technically, a League-licensed referee should also be present, but certain gym leaders from certain hometowns of people writing this post have managed to convince certain branches of the Association that a dairy farm and her farmer uncle are sufficient enough requirements for a gym battle. And because there was a precedent in one league, things just went downhill from there. Not that I’m bitter about the disorganization or anything.)

As for your questions concerning team limits, there are surprisingly few of them, thanks to the entire concept of skill matching. Even if you’re a seasoned veteran, a gym leader likely has a team that can match yours unless they’re extremely new to the job. If that’s the case, then they’re required to inform you and give you the option of taking them on as-is, finding another gym that suits your skill level, or compiling a team that would match their highest levels. If you do take on their team with your strongest, then the badge is still legitimate according to league rules, but it’s frowned upon within trainer culture to accept such a win, as essentially wiping out a gym team effortlessly defeats the purpose of a gym challenge. For this reason, it’s usually common courtesy—not a hard rule, mind you—to use weaker gyms to help train your younger or newer pokémon.

Finally, there is actually one main rule: no legendary or mythical pokémon. It’s not because you shouldn’t own them in the first place (there are, strangely, no rules against that—although, yes, it’s still a taboo to do so); it’s more because using a legendary or mythical within a gym against gym pokémon can not only injure the entire gym team but also potentially cause severe structural damage to the gym itself. Using them in the final league tournament, however, tends to be okay so long as you’re in the finals. And possibly an idiot.

Bill. —LH

What? It’s true. —Bill