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Dexterity (Dex) measures hand-eye coordination, agility, reflexes, and balance.

Dexterity is the key ability for Acrobatics, Stealth, and Thievery skill checks.

Ranged basic attacks are based on Dexterity, except with heavy thrown weapons.

A character's Dexterity modifier contributes to initiative checks.

In addition, a character's Dexterity modifier or Intelligence modifier, whichever is higher, will contribute to Reflex defense. If the character wears light armor, his or her Dexterity modifier or Intelligence modifier, whichever is higher, also contributes to Armor Class.

Dexterity and Intelligence are redundant with each other for the purpose of defense bonuses. For this reason, Dexterity may be less useful for characters who need Intelligence for attack rolls, class features, or skill checks.

Classes with Dexterity as a primary key ability
ranger (ranged original ranger, hunter, or scout)
wizard (bladesinger)
Classes with Dexterity as a secondary key ability
avenger (Wis primary, Censure of Pursuit)
druid (Wis primary, Primal Predator original druids or Circle of Shelter sentinels)
fighter (Str primary, Brawler Style, Combat Agility, or Tempest Technique)
shaman (Wis primary, Watcher Spirit)
sorcerer (Cha primary, Storm or Wild Magic)
warlock (Con or Cha primary, Fey or Gloom Pact hexblade, or Gloom Pact binder)
wizard (Int primary, Wand of Accuracy arcanist)
Races with a bonus to Dexterity
bugbear, half-orc, thri-kreen (Dex/Str)
bullywug, gnoll, halfling, half-orc, kobold, revenant (Dex/Con)
eladrin, elf, gnome, shadar-kai (Dex/Int)
bladeling, drow, elf, githzerai, goblin, hengeyokai, shifter (razorclaw), thri-kreen, wilden (Dex/Wis)
changeling, drow, goblin, halfling, hengeyokai, kenku, pixie, revenant, satyr, shade, vryloka (Dex/Cha)