Type effectiveness

Every Pokémon move has a type, and every Pokémon has one or two. The type chart rates each attacking type against each defending type: ×2, ×½, ×0, or ×1 if unlisted^1,2. A dual-type defender takes both matchups, multiplied³. The pret disassemblies and decompilation make the code of Pokémon Red and Blue, Pokémon Crystal, and Pokémon Emerald readable.

#How it works

The chart lists only non-neutral pairs, as (attacking type, defending type, multiplier) rows². Red/Blue’s has 15 types, 82 rows²; Crystal’s and Emerald’s have 17 types, 108 + 2 rows⁴. Multipliers are integers scaled by 10: 20 · 10 · 05 · 00 (÷10)¹.

In Generations 1–3, the chart runs after the base damage formula and before the random factor^5,6,7. The same-type attack bonus (STAB) comes first: $d+\lfloor d/2\rfloor$⁸ in Red/Blue and Crystal⁹, $\lfloor d\times 15/10\rfloor$¹⁰ in Emerald. The two forms agree for every non-negative integer¹⁰. Each matching row then costs one multiply and one integer divide, $\lfloor d\times m/10\rfloor$ per row^3,11,12.

damage = base_damage(level, power, attack, defense)        # see damage-formula
if move.type in attacker.types:                            # STAB, before the chart
    damage = damage + floor(damage / 2)                     # Gen 3 writes floor(damage * 15 / 10)
for (atk, def, m) in TYPE_CHART, in table order:
    if atk == move.type and def in defender.types:
        damage = floor(damage * m / 10)                     # truncates after every row
        Gen 1:      if damage == 0: the move misses
        Gen 2 / 3:  if damage == 0 and m != 0: damage = 1
damage = random_factor(damage)

Interactive: type-matrix (needs JavaScript)

#Truncation makes row order matter

A net-neutral hit whose ½ row precedes its 2× row loses 1 point on odd damage¹³. Ice against Water/Flying goes 3 → 1 → 2^13,14,15. The probe lists every such attacker and type pair, per game.

#Implementations

#Red and Blue (Generation 1)

Red and Blue¹⁶ do it all in AdjustDamageForMoveType: STAB, then a walk of the full TypeEffects table³. Single-type Pokémon store their type twice, but each row applies once³.

• Zero means miss. A row that leaves 0 damage flags a miss, which is also how ×0 works³. Under two ½ rows (×¼), damage goes 2 or 3 → 0 (miss)¹⁷.
• The message follows the last row. Each matching row overwrites one effectiveness byte¹⁸. Ice against Water/Flying, net ×1, prints “It’s super effective!”¹⁹. Ground against Rock/Flying does no damage but prints the plain miss text, not “It doesn’t affect…”²⁰.
• Fixed-damage moves skip the chart. Sonic Boom, Seismic Toss, Dragon Rage, Night Shade, Psywave, and Super Fang bypass the formula and the type routine²¹.
• The trainer AI reads one row, the first matching either defender type, so it ignores 4× weaknesses and canceling pairs²².
• Type picks the stats. Fire, Water, Grass, Electric, Psychic, Ice, and Dragon moves use Special on both sides; the rest use Attack against Defense²³.
• An unused type. The constants define a BIRD type that no row mentions²⁴.

#Crystal (Generation 2)

Crystal²⁵ adds Steel and Dark²⁶. The work moves into BattleCommand_Stab, which applies weather and badge boosts before STAB⁹. Struggle skips the routine²⁷.

• Immunity is explicit. A ×0 row marks a miss²⁸. Any other step that truncates to 0 gives 1, so ×¼ no longer misses: 2 → 1 → 1²⁹.
• The message uses the whole product³⁰, so Ice against Water/Flying prints nothing³¹.
• Foresight. The Normal→Ghost and Fighting→Ghost immunities follow a marker row, where the loop stops once the target is identified³².
• Steel and Dark keep the type-based split: Steel is physical, and Dark is special³³.

#Emerald (Generation 3)

Emerald³⁴ keeps Crystal’s chart row for row³⁵, as a flat byte array with the Foresight marker as its own row^36,37. The arithmetic is unchanged¹².

• Messages come from flags. A ½ row and a 2× row cancel, and a ×0 row sets “doesn’t affect”³⁸.
• Abilities act as extra immunities. Levitate makes Ground moves miss, so the loop reads no row³⁹. Wonder Guard lets only super-effective hits through⁴⁰.
• Carried over. Struggle skips STAB and the chart⁴¹. Type decides physical or special⁴². The unused slot becomes TYPE_MYSTERY, “???”⁴³.

#What changed, Gen 1 → Gen 3

	Red/Blue	Crystal	Emerald
Types	152	17, adds Steel and Dark26	1736
Matchups changed	—	4, listed below44	none35
Step that reaches 0	move misses3	raised to 1; ×0 is a miss11	raised to 1 unless ×012
Effectiveness text	last matching row18	combined value30	flags, ½ and 2× cancel38
Immunity removers	none	Foresight32	Foresight37
Random factor	×217–255 / 25545	×217–255 / 25546	×85–100 / 10047

A cell-by-cell diff of the Red/Blue and Crystal tables gives +Steel, +Dark, 4 changed⁴⁴. The four changes are Poison→Bug ×2 → ×1, Bug→Poison ×2 → ×½, Ghost→Psychic ×0 → ×2, and Ice→Fire ×1 → ×½⁴⁴. Bulbapedia lists the same four⁴⁴.

#Ghost and Psychic in Generation 1

Red and Blue’s table gives Ghost moves ×0⁴⁸ against Psychic types, but a Saffron Gym trainer says “Psychic #MON fear only ghosts and bugs!”⁴⁹. Crystal’s table gives ×2⁵⁰. Bulbapedia reports that “an abundance of evidence suggests that this may have been an error”⁵¹.

#Variants and combinations

• STAB runs in the same routine, directly before the chart, in all three games^8,9,10.
• Damage formula: in Gen 1–3, type also makes a move physical or special^23,33,42.

#History

The games read here run from Red and Green (Japan, 1996) to Emerald (2004)^16,34.