Note the use of red, white, and green dice for physical, mental, and spiritual attributes.
First level characters begin with one physical (red) hit dice; losing all the hit points from this die represents sustaining lethal wounds or being knocked unconscious. A poisoned weapon has to deal physical damage to force a save vs. poison.
Second level characters begin with one spiritual (green) dice; losing all the hit points from this die represents losing esprit de corps and becoming fatalistic.
Third level characters begin with one mental (white) dice; losing all the hit points from this die represents running out of plans and ideas for how to handle the situation.
At higher levels, characters gain additional dice in each category, beginning with the type that represents their class's primary statistic.
A simple way to think about hit dice is like this: A PC has 3 hit dice (all OD&D hit dice are d6) and no Con bonuses to HP. They go into a dungeon and take 17 points of damage. Do they survive?
- Only if they rolled triple 6's when they rolled up / levelled up this character. (Most common interpretation, canonized by later editions, with variants on whether when you level up you re-roll the dice you previously rolled or are stuck with the results of the previous levels' rolls.)
- Only if they rolled triple 6's at the start of that day's adventure. (A rarer take on OD&D's failure to explain how hit dice work, which Jeff Rients likes because it encourages players to say "Hmm, despite his 6 hit dice it looks like Goshan the Invincible feels like crap today. He's going to stay at the inn and get drunk. Obol the Henchman, who rolled double 6's on his two hit dice, will go for the glory this time!")
- Only if they roll triple 6's at the moment in which they're damaged, moving through each dice roll in order for maximum drama and imaginative potential. (This is the method I use, inspired by Zulgyan's method of rolling monster HD only when they're hit, and leaving the dice on the table next to the monster as a way to continue subtracting how many hit points remain after damage is taken.) If damage blows through the hit points you got from your mental HD, your plan has failed and you're running by the seat of your pants. If all your hit points from spiritual HD are gone, you're demoralized. If all your physical hit points are gone, you're dead.
Healing works as follows:
- At any one time, each of your hit dice is in one of three states: untouched and thus untested, damaged - which means you now know how many hit points you have from that hit dice - or exhausted, which means you already rolled that dice and lost all the hit points it granted you.
- If tending your wounds is the first thing you do after the fighting is over, you receive 1d4 points of healing. What this means depends on which of your dice is damaged - bandaging wounds (physical), drinking or praying (spiritual), or reviewing tactics and sharpening swords (mental).
- Healing is only going to affect damaged hit dice. It won't affect untouched hit dice, except that once you receive an amount of healing that equals or exceeds the total amount of damage you've taken, all your hit dice return to being a blank slate. And it won't restore points from an exhausted dice; once you lose your mental and spiritual reserves, you can't get them back until you're fully healed, which means you're vulnerable to poison (to use the above example).
For example, if someone hits you with a cure light wounds and rolls 7 for the amount cured, what happens is:
- If you've taken 7 or less points of damage, you go back to being entirely fresh (all your hit dice are untouched).
- If not, two things happen. You subtract 7 from the total amount of damage you've taken, and you add 7 to whatever type of die is currently damaged. In the example of the guy who survived the 17 points of damage with 1 physical hit point remaining, he now has 8 physical HP.