Base Defense Bonus represents a character's martial skill at avoiding blows and parry blows, and roll with them to have armor or gear absorb most of it.

The Defense Bonus is a dodge bonus, but only stacks with armor if the character has proficiency in the armor type. Any time you would be deprived of your dexterity modifier to armor class you also do not gain the benefit of the defense bonus.

Base Defense Bonus is determined by class and level. Multi-classed characters stack their base defense bonuses of their class. Thus, a second level rogue/ranger would have BDB of +2.

Prestige classes have their own rates of defense bonus advancement that is listed with their class information.

NPC Classes gain base defense as follows: Warriors as fighters, commoners and experts as wizards.

Level
Aristocrat
Barbarian
Bard
Fighter
Monk
Paladin
Priest
Psioncist
Ranger
Rogue
Wizard
Witch
1
+0
+1
+0
+0
+1
+0
+0
+0
+1
+1
+0
+0
2
+1
+1
+1
+1
+1
+1
+0
+0
+1
+1
+0
+0
3
+1
+2
+1
+1
+2
+1
+1
+0
+2
+2
+0
+0
4
+1
+2
+1
+2
+2
+2
+1
+1
+2
+2
+1
+1
5
+2
+2
+2
+2
+3
+2
+1
+1
+2
+2
+1
+1
6
+2
+3
+2
+3
+4
+3
+2
+1
+3
+3
+1
+1
7
+2
+3
+2
+3
+4
+3
+2
+1
+3
+3
+1
+1
8
+3
+3
+3
+4
+4
+4
+2
+2
+3
+3
+2
+2
9
+3
+4
+3
+4
+5
+4
+3
+2
+4
+4
+2
+2
10
+3
+4
+3
+5
+6
+5
+3
+2
+4
+4
+2
+2
11
+4
+4
+4
+5
+6
+5
+3
+2
+4
+4
+2
+2
12
+4
+5
+4
+6
+7
+6
+4
+3
+5
+5
+3
+3
13
+4
+5
+4
+6
+7
+6
+4
+3
+5
+5
+3
+3
14
+5
+5
+5
+7
+7
+7
+4
+3
+5
+5
+3
+3
15
+5
+6
+5
+7
+9
+7
+5
+3
+6
+6
+3
+3
16
+5
+6
+5
+8
+9
+8
+5
+4
+6
+6
+4
+4
17
+6
+6
+6
+8
+9
+8
+5
+4
+6
+6
+4
+4
18
+6
+7
+6
+9
+10
+9
+6
+4
+7
+7
+4
+4
19
+6
+7
+6
+9
+10
+9
+6
+4
+7
+7
+4
+4
20
+7
+8
+7
+10
+12
+10
+6
+5
+8
+8
+5
+5


Monsters

Monsters gain the benefit of Base Defense Bonus as well. The rate of BDB advancement is based on the monster type and hit dice.
  • Aberration - as fighter
  • Animal - as rogue
  • Beast - as fighter
  • Construct - as priest
  • Dragon - as fighter
  • Elemental - as priest
  • Fey - as rogue
  • Giant - as fighter (or by class)
  • Humanoid - by class
  • Magical Beast - as fighter or rogue (let your conscience be your guide)
  • Monstrous Humanoid - as fighter (or by class)
  • Ooze - as wizard
  • Outsider - as fighter
  • Plant - as wizard
  • Shapechanger - as rogue
  • Undead - as priest
  • Vermin - as rogue

It is important to note that that the above is just a guideline, and some specific monsters might be better off with a different kind of advancement. For example, some free-willed undead could be treated as a fighter or rogue.

See also, Combat, Armor, Classes, Prestige Classes