Type chart
Type chart is the most important tool because shows which modifiers are applied to move types when attacking Pokémon of each type.
Represent
Overall
Multi-type
1) When Psychic-type move------>Ground & Steel-type Pokémon
Psychic------>Ground=1
Psychic------>Steel=0.5
1×0.5=0.5, Not very effective.
2) When Fire-type move------>Grass & Rock-type Pokémon
Fire------>Grass=2
Fire------>Rock=0.5
2×0.5=1, Still OK.
3) When Fighting-type move------>Ghost & Ice-type Pokémon
Fighting------>Ghost=0
Fighting------>Ice=2
0×2=0, Doesn't affect...
4) When Ground------>Fire & Steel-type Pokémon
Ground------>Fire=2
Ground------>Steel=2
2×2=4, Super effective!