Tuthrick: This was also quite intriguing to me, but it was with Montaron and his HP for each class. I decided to do some tests for Jaheira's case. I've installed 'max HP per level up' mod for convenience and here's what I discovered (it kind of proved what I previously thought, but one case is interesting):
I created 3 characters with 17 Constitution: Fighter/Thief, Fighter/Mage and Fighter/Cleric and used cheat codes to gain exact experience points to level up in only one class, then the other and do so again for level 3 for both classes. Each time the characters gained the same amount of HP for given class.
1) Fighter/Thief started with 11 HP total at level one. For Thief class he got 4 HP for each level up, doesn't matter if it's level 2 or 3. For Fighter class he got 7 HP for each level up. So, for level 2/2 he has 22 HP, for level 3/3 - 33 HP and so on.
2) Fighter/Mage is a weird case, he started with 10 HP. I thought that since it's the same Con, he would get 7 for Fighter and 3 for Mage, but to my surprise he got 6 for Fighter and 4 for Mage each time. So, for level 2/2 he has 20 HP and for 3/3 - 30 HP and so on.
3) The last example is the same as Jaheira, because Clerics get the same amount of HP as Druids, but they gain levels faster than fighters, so I can see how many does she get for each class separately. The character started with 12 HP. For Cleric class he got 5 HP for each level up and for Fighter he got 7 HP for each level. So, for level 2/2 he has 24, for level 3/3 - 36 and so on.
I don't think that the 'max HP per level' has influenced the results, because it only changes each roll to be max possible. The mechanics itself should be the same.
I always thought that the max possible HP for each level is the same as HP on level 1, no matter what level the multiclassed character advances to. It seems to me that the bonus for Con is divided between the classes, especially since in that second character got 4 HP for Mage and it's just strange to me. I remember that when a character advanced in both classes at the same time it was almost impossible to get the max possible HP total, it seems that it's just two separate rolls and therefore chance for getting max HP is very small.
I can explain how this happened.
1. Fighter/Thief gained 5 base HP per Fighter level and 3 per Thief level. You see 6 HP on Fighter level up because the character's average level increased to 1.5, increasing the character's Con HP from 6 to 7.5 (a 1.5 HP increase), which the game rounds dow to 1; hence 6 HP. Then, on gaining a Thief level, this character's Con HP increased to 9, giving them 2 Con HP, for a total of 5 HP gained.
2. The HP gains from fighter levels are actually the same here; however, since Fighter is gained first, the character gains 1.5 Con HP, which the game rounds down; hence, the character gets only 6 HP for this level. After gaining a Mage level, however, the character gains the other 1.5 Con HP, which raises it to a whole number, giving them a total of 4 HP from the level up.
Case 3 has you gaining the non-Fighter level first again, so you get the Con bonus rounded up with those levels.
Jaheira actually won't behave the same way. Try this test with a Fighter/Druid, and compare the HP gains on reaching Druid 7 to those on reaching Druid 8; you should get different HP gains for those two levels. (The pertinent fact here is that the character will get both these levels after Fighter 6, but before Fighter 7.)
Also, this irregularity will disappear if you use a character with an even Con bonus.
If you look at the character in a character editor (something like ShadowKeeper or EE Keeper), you can see the character's base HP, which should always increase the same amount at every level up with max HP gain.
By the way, for a triple class character with no Con bonus, the character should gain 3 HP per Fighter level, 1 per Mage level, and 2 per Thief or Clerice level. (This is assuming that Hickory is wrong about base HP lost from rounded down being carried to the next level; if Hickory is correct, then you will get higher numbers for some level ups, so this can be used as a way to test that claim.)
Tuthrick: 3) The last example is the same as Jaheira, because Clerics get the same amount of HP as Druids, but they gain levels faster than fighters, so I can see how many does she get for each class separately. The character started with 12 HP. For Cleric class he got 5 HP for each level up and for Fighter he got 7 HP for each level. So, for level 2/2 he has 24, for level 3/3 - 36 and so on.
bushwhacker2k: So the result was that a Fighter/Cleric with 17 Constitution got 12 hp per collective level? That doesn't seem in line with the idea that the Constitution bonus is halved for both classes, since that'd be (10+8+3+2) / 2 = 11.5.
I was actually thinking that maybe for character creation they just take the Warrior Constitution bonus (3 for 17) added to 9 to get 12, then after that the Constitution bonus is halved for both classes... but if you can actually get 12 every time, that doesn't seem to be true...
I think I'm getting more confused and less sure as this goes on...
As I said, the Constitution bonus is internally separate from base HP, and is recalculated from scratch at every level up. Hence, the character is getting 9 base HP per collective level (5 from fighter and 4 from cleric), and the constitution bonus HP is recalculated every time. (It's also recalculated if the character's Constitution changes mid-game; try having the character cast Draw Upon Holy Might, for example.)
