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Constitution represents your character's health and stamina.  The constitution
ability score bonus impacts a character's hit points, so constitution is
important for all classes.
 
Changes to your constitution ability score bonus now apply retroactively to
all the levels you have gained, so such a change will have a significant
impact on the hit point count of a high-level character.
 
You apply your character's constitution modifier to:
 
- Each roll of a hit die: Every time you gain a level, the game rolls a hit die
  corresponding to your class.  The constitution modifier is applied to this
  roll, increasing or decreasing it.
 
- Your fortitude saving throw, used in saving against poison, unconsciousness,
  disease, debilitating blows, and similar effects.
 
- Concentration checks, used to determine whether your spell caster is able to
  complete their spellcasting despite being injured in combat.  Important for
  spellcasters when they are involved in combat
 
- Any other skill or trade that has Constitution (CON) as a primary attribute
 
Average Constitution Scores
 
Example Race or Creature      Avg Con       Avg Mod
Ghoul, Mummy                     -             -
Centipede, Locust               8-9           -1
Human, Imp, Grick              10-11          +0
Medusa, Nymph                  12-13          +1
Light Horse, Merfolk           14-15          +2
Tiger, Chimera                 16-17          +3
Polar Bear, Umberhulk          18-19          +4
Elephant, Aboleth              20-21          +5
The Tarrasque                   35            +12