Terastallization does two separate things at once, and conflating them is the most common mistake newer players make with the mechanic: it changes a Pokémon's offensive STAB calculation, and it independently overwrites the Pokémon's defensive typing entirely. Understanding those as two separate systems — one additive, one a full replacement — is what separates a Tera call that wins a game from one that just changes a type for no functional reason.
The Offensive Side: STAB Stacking
Before Terastallizing, a Pokémon gets the standard 1.5x STAB bonus on moves matching either of its original types. After Terastallizing into a type that matches one of its original types, that bonus increases from 1.5x to a full 2x — Tera doesn't add a second STAB source, it upgrades the existing one. If a Pokémon instead Terastallizes into a type that doesn't match either original type, it gets a fresh 1.5x STAB on moves of the new Tera type on top of still getting 1.5x STAB on moves matching its original types, meaning a Pokémon with strong coverage in both its original and Tera type effectively gets two separate STAB categories to attack from in the same battle.
The Defensive Side: A Full Typing Overwrite, Not an Addition
This is the part that trips people up: Terastallizing does not add the Tera type as a second defensive type. It replaces the Pokémon's defensive typing entirely, so a Terastallized Pokémon is calculated as pure single-type for every incoming attack, regardless of how many types it had before. A Water/Ground Pokémon that Terastallizes into Grass stops being weak to Ice and stops resisting Electric — none of its original defensive interactions carry over, because for defensive purposes it is now simply a Grass-type, full stop. This is what makes Tera such a strong defensive tool: it can delete an existing weakness completely rather than just mitigating it, provided the new single type doesn't introduce a worse one.
Case Study: Tera Grass Heatran
Base Fire/Steel Heatran is weak to Ground and Water despite an otherwise excellent resistance count, and both are common, expected coverage types aimed specifically at Fire/Steel Pokémon. Terastallizing into Grass overwrites that dual typing with pure Grass for defensive purposes: Grass resists Ground and Water outright, converting Heatran's two most dangerous incoming attack types into resisted hits in the same turn, while Heatran still gets to attack with Fire and Steel STAB moves at the original 1.5x rate since neither matches the new Tera type. The trade-off is that Grass introduces fresh weaknesses to Fire, Ice, Poison, Flying, and Bug that Heatran didn't have before — but if the opponent's team is built around punishing Heatran specifically with Ground or Water coverage, none of those five newly-introduced weaknesses may even be present on their side.
Stellar Tera: The Thirteenth Type
Stellar is a Tera-exclusive type that isn't available on non-Terastallized Pokémon and works differently from every standard Tera type. Terastallizing into Stellar gives a Pokémon a 1.2x boost (weaker than a normal Tera STAB boost) on every move type it knows, but that boost applies only once per move type for the rest of the battle — the first time a Stellar-Tera Pokémon uses a Fire move, it gets the 1.2x bump, but every subsequent Fire move that battle reverts to normal, non-Stellar-boosted damage, while a different move type used for the first time still gets its own one-time 1.2x bump. Defensively, Stellar Tera does not grant any resistances or immunities the way a standard single-type overwrite would — a Stellar-Terastallized Pokémon takes damage from all eighteen types at their original, pre-Tera multipliers, since Stellar itself has no defined chart interactions. Stellar is exclusively available to Terapagos and to Pokémon that haven't yet Terastallized once in a given format's specific ruleset allowances, making it a build-around rather than a default option.
Case Study: Tera Normal Dragonite
Dragonite's signature move Extreme Speed is Normal-type priority, and Dragonite's own typing (Dragon/Flying) means it normally only gets neutral, non-STAB damage from it. Terastallizing into Normal grants a full 1.5x STAB bonus on Extreme Speed for the first time, turning an already-useful priority move into a genuine, hard-hitting finisher that outpaces the entire format by virtue of moving before slower threats regardless of Speed stat. The defensive trade-off is real — pure Normal Tera loses Dragonite's resistances to Fire, Water, Grass, and Electric and picks up a Fighting weakness it didn't have as Dragon/Flying — but the calculation is usually made specifically as a late-game cleaning tool where Dragonite is expected to attack once and win, not to tank a hit afterward.
Case Study: Tera Water Urshifu-Rapid-Strike
Urshifu-Rapid-Strike's signature move Surging Strikes is Water-type, hits three times per use, and each hit is a guaranteed critical hit — critical hits ignore the target's positive stat stage changes and calculate damage as though the attacker's own beneficial stages are active, but they don't ignore STAB or type effectiveness. Terastallizing into Water upgrades Surging Strikes' existing 1.5x STAB to a full 2x across all three hits, compounding with the guaranteed-crit mechanic into some of the highest sustained physical damage output available on a single move in the format. Defensively, pure Water Tera trades Urshifu's original Fighting/Water resistances and weaknesses for Water's own profile (resists Fire, Water, Ice, Steel; weak to Grass, Electric), which usually nets out favorably since it removes the Fairy and Flying weaknesses Fighting/Water carried while keeping most of the same resistance count.
Choosing Offensive vs. Defensive Tera at Teambuilding Time
The decision comes down to what the Pokémon's role actually needs from the mechanic: an offensive Tera call (matching an existing type for the 1.5x-to-2x STAB upgrade, or picking a new type for fresh coverage) is the right choice on a Pokémon whose win condition is dealing damage in one big turn, while a defensive Tera call (overwriting a bad weakness with a favorable single type) is the right choice on a Pokémon whose job is surviving a specific predicted attack to set up or pivot. Both uses spend the same single-use-per-battle resource, which is exactly why Tera prediction — guessing whether the opponent is about to Tera offensively to break through a wall or defensively to survive a lethal hit — has become one of the most contested mental layers of Gen 9 team preview and in-game decision-making.
Tera Blast and the Coverage-Only Use Case
Tera Blast is a move that takes on the user's current Tera type when used, meaning a Pokémon that would otherwise have no way to attack with its Tera type at all can still get STAB off of it through this one move alone. This matters most for defensive Tera calls: a Pokémon that Terastallizes purely to survive an attack and change its defensive typing doesn't need a moveset built around the new type, since Tera Blast covers the offensive gap on its own, freeing the other three moveslots to stay focused on the Pokémon's original role rather than being redesigned around a type it only adopted reactively.
Reading the Opponent's Team Preview for Tera Signals
Because Tera type isn't revealed until the moment a Pokémon actually Terastallizes, team preview forces both players to play around an unknown variable for the entire early game — a Pokémon with an obviously exploitable weakness (a Water/Ground-type facing a team with double Grass coverage, for instance) is a strong signal that its Tera type was likely chosen specifically to patch that weakness away, and experienced players adjust their opening lines accordingly rather than assuming a team's on-paper weaknesses will still apply once Tera is spent. This unknown-variable pressure is deliberate design: Game Freak built Terastallization specifically to punish players who plan a full game around team preview information alone without leaving room for a single-use typing change to invalidate that plan mid-battle.