Poison

Poison pressures Grass and Fairy, and often resists those same types when defending.

Look up the type matchup matrix View defense in the matrix

Two-way summary

Dealt by Poison

×2 Increased (×2): Grass
Fairy
×½ Reduced (×½): Poison
Ground
Rock
Ghost
×0 Immune (×0): Steel
×1 Neutral (×1): 11 neutral types — open the full table below

Taken by Poison

×2 Increased (×2): Ground
Psychic
×½ Reduced (×½): Grass
Fighting
Poison
Bug
Fairy
×1 Neutral (×1): 11 neutral types — open the full table below

×2 Increased (×2)
×1 Neutral (×1)
×½ Reduced (×½)
×0 Immune (×0)

How does the second type change things?

Normal moves

Arbok

Arbok

Poison

Multiplier on Poison: ×1
Result: ×1

Gengar

Gengar

Multiplier on Ghost: ×0
Multiplier on Poison: ×1
Result: ×0