Same-type attack bonus

Same-type attack bonus is a 50% boost to an attack's power when the attack's type is the same as one of the types of the Pokémon using the attack. For instance, if a Blastoise were to use Surf, Surf's power would be increased by 50% since both Blastoise and Surf are water-type. If Blastoise were to use Ice Beam however, it would receive no such bonus since Blastoise is not ice-type. A Lapras on the other hand, would receive the same-type attack bonus when using either Surf or Ice Beam since Lapras is both ice-type and water-type.

Contrary to some of Nintendo's official guides, normal-type Pokémon do in fact receive the same-type attack bonus when using normal-type attacks, and same-type attack bonus is not reduced for a Pokémon with two types.

Hidden Power does receive same-type attack bonus. A fire-type Pokémon will receive same-type attack bonus when using a fire-type Hidden Power. Although Hidden Power is normal-type in the game's data, a normal-type Hidden Power is not really obtainable. Because of this, a normal-type Pokémon will never receive same-type attack bonus when using Hidden Power.

It should also be noted that due to the game treating Hidden Power like a Normal-type attack, Mirror Coat will not work against any Hidden Power, but Counter always will.