Badge Boosts

In generations 1 through 3, some gym badges give you certain boosts that make your pokémon more powerful.

Generation 1

Each badge gives you a boost to the appropriate stat by a factor of 9/8. The boosts are described in the following table:

Attack	Boulder Badge (Brock)
Defense	Thunder Badge (Surge)
Speed	Soul Badge (Koga)
Special	Volcano Badge (Blaine)

Note that the game tells you that the Thunder Badge boosts speed and the Soul Badge boosts defense. The game is wrong.

Stacking Badge Boosts

Whenever one of your stat levels gets modified during a battle, the game recalculates that stat based off of the unboosted value, then applies all of your badge boosts. That means that the other stats get an extra copy of their badge boost. The stacked badge boosts go away whenever the stat is recalculated, meaning whenever that stat is modified, you level up, or the battle ends.

For example, if you have the Boulder Badge and you get hit by a string shot, lowering your speed, then the game sets your speed to 2/3 of your original speed stat, then applies all badge boosts, increasing your attack by a factor of 9/8. The same effect can be seen when using a stat boosting XItem (not XAccuracy). Since it raises the level of one of your stats, all of the other stats will badge boosts will get another boost by 9/8. This can be seen in the Champion fight in Yellow, where an XSpecial gives Nidoking the defense to survive Sandslash's earthquake.