Slightly more complicated than that.
When Supplement II introduced the monk and assassin classes late in '75, these were the first classes that had a built-in level cap for human characters but which got one hit die per level all the way up to their cap (rather than transitioning from hit dice to bonus hp the way fighting men, magic-users, clerics, and thieves all did). The monk had 16 levels and got 1d4 hp per level (max 1d6d4), and the assassin had 13 levels (not counting the pseudo-14th level of "guildmaster," which didn't get a hit die) and 1d6 hp per level (max 13d6).
This is in comparison to the four basic classes, which had no upper limit on levels, but hit dice capped out as follows: 9d8 for fighting men (+2 hp per level above 9th), 11d4 for magic-users (+1 hp per level above 11th), 8d6 for clerics (+1 hp per two levels above 8th), and 10d4 for thieves (+1 hp per two levels above 10th). The cleric and the thief only gained a bonus hit point every other level above name level because fighting men and magic-users needed considerably more XP to gain levels. (In AD&D, this discrepancy was resolved by increasing the XP needed for clerics and thieves to reach high levels, brining them in line with white box OD&D fighters and magic-users. Whereas in basic D&D, the opposite was done, and fighters and magic-users needed less XP above name level, more in line with the original cleric and thief.)
So in AD&D, you had nearly all the classes other than magic-user get their hit points bumped up.
• Fighters now got 9d10 (+3 per level after), clerics 9d8 (+2), thieves 10d6 (+2), and magic-users 11d4 (+1).
• Paladins had the same hit dice as fighters (which made sense, since paladins began as a set of tacked-on abilities for Lawful fighters with Cha 17+); whereas rangers (which originally, in
The Strategic Review, had 2 hit dice at 1st level and went up to 10 dice at 9th, +2 thereafter; but could have
either d6 or d8 dice depending on whether the DM's campaign was using the original LBB rules for hit dice where everyone had d6s, or the revised rules from Sup I that gave fighters d8s) were settled on d8s for hit dice, 2 at 1st level and then up to 11d8, still +2 above that.
• Illusionists were pretty much like MUs, except that they capped out at 10d4 instead of 11d4 dice (whereas the original illusionist from SR didn't specify beyond saying "sub-class of magic-user").
• Druids had 14 levels, and so (like the monk and assassin) they wound up with one per level — in this case, 14d8 hp (bumped up from the original 13d6 in Supplement III; both versions' highest levels were still called "The Great Druid," because Grand Druids and Hierophants wouldn't be a thing until
Unearthed Arcana; but AD&D inserted the "Ovate" rank at 2nd level between Aspirants and 1st Circle Initiates).
• Assassins now had 15 levels (with "guildmaster" becoming a real 14th level, and "grandfather of assassins" the new 15th level, so up to 15d6 for hit dice).
• And bards, originally 1d6 per level up to 10d6 (+1 hp thereafter) were changed into that weird prestige class, apparently able to earn those 10 six-sided hit dice on top of any fighter or thief hit dice acquired first.
Which brings us to the monk. The Supplement II monk was very clearly noted a sub-class of cleric, same as the druid. If you had all the supplements and newsletters, the classes in OD&D were fighting man (paladin, ranger), magic-user (illusionist), cleric (monk, druid), thief (assassin), and bard. AD&D pulled the monk out and made it its own class, gave it an extra experience level (just like the assassin and the druid), and also gave it an extra hit die at level 1, the same as the ranger had. So AD&D monks had 17 levels and could have up to 18d4 for hit points, but with each die getting its own Con adjustment (and monks needed at least Con 15+ to qualify for the class in AD&D, so if you were a monk, you had at least +1 hp per die from Con, and very possibly +2 per die if your Con was 16 or better!).
This was the state of things for early 1st edition; of course late 1st edition was a different beast thanks to
Unearthed Arcana and
Dragon Magazine. I won't go into all the changes wrought on the classes by UA (like druids raising their level limit but not their hit dice, or the introduction of the barbarian), but I'll point out that the
Dragon #53 variant monk was very popular, and it raised the class's level limit from 17th to 21st and bumped the hit dice to d6s, so this monk capped out with 22d6 hit dice!
There's also basic D&D to consider, which reintroduced a version of the monk with a human monster entry called the "mystic," which could have anywhere from 1 to 16 (eight-sided) hit dice (just like the original Sup II monk's 16 levels) in both its original form and as an optional PC type in the
D&D Master Set. When the mystic was revised for the
Rules Cyclopedia, it retained its limit of 16 experience levels, but now its hit dice were six-sided like a cleric's and capped at 9 just like all the other classes. (In basic D&D, the fighter gets 9d8, +2 thereafter; the mystic, 9d6, +2; the cleric, 9d6, +1; the thief, 9d4, +2; and the magic-user, 9d4, +1 thereafter. Dwarves get 9d8, then +3 for levels 10–12; elves get 9d6 and then either +1 or +2 at 10th level depending on the version; and halflings cap out at 8th level, so they only ever get 8d6.)
When AD&D 2nd edition came along, it did a big thing: it reorganized all the sub-classes into class groups, and it
made all the hit dice uniform within a group (for the most part). When the 2nd edition monk finally appeared in
Greyhawk: Scarlet Brotherhood, it was once again a member of the "priest" group. So 2nd edition classes looked like this:
• Warriors (fighters, paladins, rangers, and I think there was even a gladiator class in this group eventually?) all got 1d10 up to 9th level, then +3 hp. (Barbarian fighters were the
sole exception, getting d12 dice up to 9th level, which was one more than the 8d12 that barbarians originally got back in UA!)
• Priests (clerics, druids, monks, crusaders, shamans, countless other specialty priests) got 1d8 up to 9th level, then +2 per level.
• Rogues (thieves, bards, assassins, ninjas) got 1d6 up to 10th level, then +2 per level.
• Psionicists got 1d6 up to
9th level, then +2 per level.
• Wizards (mages, the eight school specialists, setting-specific variants like the sha'ir, and countless more types of specialist wizards from later sources—artificers, geomancers, and so forth) all got 1d4 up to 10th and then +1 hp thereafter, just like the 1st edition
illusionist (one fewer than the 1st edition magic-user!).
So when 3rd edition came around and obliterated the concept of sub-classes or class groups, the days of this beautiful uniformity were numbered. It held on for a while: with the original PHB classes, you could see a reflection of the old class groups in fighters, paladins, and rangers all having 1d10, the barbarian uniquely still having its d12, the cleric and druid and monk all sharing the d8, the rogue and the bard with d6, and the wizard and sorcerer with d4.
3.5 came along, dropped the ranger back down to d8, and that was pretty much the end of the old system. Whatever new classes were added in 3.5, any hit die type was fair game, and there was no more real logic to it. 4th edition, of course, did away with hit dice entirely, and when 5th edition brought them back, it took a cue from
Pathfinder in excising the d4 hit die from the list, which resulted in another system-wide bump of hit points across many of the classes — but one that worked exactly
opposite to the original bump that Gary Gygax worked into 1st edition, to the advantage of fighters and the necessary detriment of magic-users.
So… yeah. Thanks for coming to my blog talk. ∎