Simply put, critical hits did not exist as official rules in D&D for a long time. The reasoning was that it was outside of what Gygax thought was appropriate for what D&D’s hit point/attack roll/combat system was supposed to represent, especially when you combined it with his insistence that both NPCs and players must follow the same rules:

From the AD&D 1st Edition DMG:

Combat is a common pursuit in the vast majority of adventures, and the participants in the campaign deserve a chance to exercise intelligent choice during such confrontations. As hit points dwindle they can opt to break off the encounter and attempt to flee. With complex combat systems which stress so-called realism and feature hit location, special damage, and so on, either this option is severely limited or the rules are highly slanted towards favoring the player characters at the expense of their opponents. (Such rules as double damage and critical hits must cut both ways — in which case the life expectancy of player characters will be shortened considerably — or the monsters are being grossly misrepresented and unfairly treated by the system. I am certain you can think of many other such rules.)

At best, natural 20s were sometimes considered automatic hits:

In Original D&D or Holmes Basic, there was no rule for this, but the highest attack roll you’d ever need to hit was a 17, if you were attacking a target with AC 2 and you were a 1st level Fighting Man.

BECMI/Rules Cyclopedia D&D specifically states that natural 20s are considered automatic hits (and natural 1s are automatic misses)

When you get to AD&D 1e though, no such rule is in effect, and you also get attack roll matrices where you sometimes need higher than a 20 to hit.

In all other cases, critical hits were only houserules or third-party products. Although Gygax/TSR was very much aware that this was something that players did, they remained quite firm in never officially supporting them.

Our first official mention of critical hits is in AD&D 2nd Edition, where even there it is an optional rule. It mentions two systems:

1. Every natural 20 rolled while making an attack will let the character roll their damage dice twice. It’s explicitly mentioned that you should not just roll damage once and multiply it by two. Flat modifiers to damage are left as-is.

2. Every natural 20 rolled while making an attack will let the character make another attack right after that one. If that second attack also comes out to a natural 20, then a third attack will be made, and so on and so forth. The damage rolls are otherwise completely normal, and you might not even get any benefit from the natural 20 if your second attack misses.

For the first method, I want to call attention to the phrasing:

From the AD&D 2nd Edition DMG:

The simplest critical hit system makes every natural 20 rolled on the attack roll count for double damage.

Note that there is no expectation here that the natural 20 is an automatic hit, only that it will count for double damage. If you’re particularly cynical, you might read this as saying that it only counts if the natural 20 would normally result in a hit anyway, because remember that we’re still in this phase of D&D’s encounter design where you might have the Magic-User rolling attacks against a monster with negative AC and he needs a 23 to hit per the attack roll tables.

That brings us to the next step in critical hits, in AD&D 2e’s Player’s Option – Combat & Tactics

C&T presented two critical hit systems.

The second one was rather detailed and resembled the more elaborate unofficial critical hit systems that other people created:

1. If the attacker rolls a natural 18, and the margin of success of the attack roll vs the target’s AC is 5 or more, then the target makes a saving throw vs Death.

2. If the saving throw succeeds, damage is dealt as if it were a normal hit.

3. If the saving throw fails, then a critical hit happens. Roll damage dice twice, and leave flat bonuses as-is, and then proceed further for more specific injuries

4. Your weapon is either going to be slashing, piercing or bludgeoning

5. Your target is either going to be a humanoid, an animal or a monster

6. From #4 and #5, look at the correct table for your weapon versus your target, such as an animal hit by a slashing weapon

7. Roll a d10 to determine where you hit the target, such as on the torso, the leg, the arm, the head, etc

8. Roll for the severity of the hit. The dice here can be as small as a 1d6 if your weapon size is smaller than the target’s size, or as high as 2d8 if your weapon is two sizes larger.

9. Check the specific injury dealt depending on the severity roll. A 6 might cause a broken foot to reduce speed by half, a 10 might break ribs and prevent all action, and higher results can cause triple damage dice and/or instant death

It’s also worth noting that this is where we get the mention of only certain enemies being vulnerable to critical hits:

From Player’s Option – Combat & Tactics:

Some monsters are naturally resistant to the effects of certain critical injuries. Creatures such as golems, undead, or elementals don’t bleed and therefore ignore any such effects. A monster like a hydra can lose a head without being instantly slain. However, these injuries can still be important because it might affect the way a monster moves or attacks. A skeleton that’s had a leg knocked off can’t move at its full rate, even if it is less troubled by the injury than a living person would be. Slimes and jellies have no parts that are more specialized or important than the rest of the body, and are therefore immune to the effects of most critical hits. Use common sense to handle these situations as they arise.

This principle would rear its head again in 3rd Edition.

That brings us back to the first critical hit system in C&T. This one was much simpler.

From Player’s Option – Combat & Tactics:

Critical hits occur when a character rolls a natural 18 or higher and hits the target by a margin of 5 or more after all adjustments. If the character scores a critical hit, he inflicts double damage dice, calculated before adjustments for Strength, magic, or special circumstances. In a situation where the damage is doubled for another reason (such as a charge with a lance or backstabbing), all multipliers are calculated before adding Strength, magical, and other adjustments.

I described this in the previous critical hit system, but take note of how crits are triggered: natural 18+, but the margin of success over the target’s AC must be 5 or higher.

The book actually goes on to explain why the rule is structured this way: They wanted to avoid a situation where a character can only hit their target on a natural 20, and then all of those hits are also critical hits. This is important because it’s going to lead us straight into 3rd Edition, with its crit confirmations.

In D&D 3rd Edition:

1. A natural 20 is always an automatic hit

2. Your weapon is going to have a “critical threat” property, which defines which natural numbers on a d20 can trigger a critical threat. A longsword, for example, has a critical threat of 19-20, which means a critical threat is triggered on a natural 19 and 20 (automatic hits are always only natural 20s)

3. If you make an attack roll and the d20 comes up on a number that’s within the weapon’s critical threat, you make a second attack roll identical to the one you just made.

4. If that second attack roll is a miss, then it’s a normal hit

5. If that second roll is still a hit, you score a critical hit. You roll your damage dice a second time and you take your flat modifiers a second time. If your weapon has an x3 or x4 critical hit multiplier, you can even roll your damage dice and add your flat modifiers that many more times.

There’s a bunch of design considerations that would come out of this:

You can differentiate weapons more, since “threat range” and “critical multiplier” are two additional properties that you can manipulate. A kukri is a 1d4 light melee weapon, same as the dagger, but the kukri has a critical threat of 18-20 to the dagger’s 19-20, but then the dagger can be thrown. Meanwhile a light pick is another 1d4 weapon and has a threat range of just 20, but has a 4x critical multiplier.

It’s also another avenue for character progression, since you can have feats, spells and class abilities that manipulate your threat range and your multiplier. One easy example is the Warblade getting a bonus that specifically only applies to its crit confirmation attack rolls. That said, this can also turn ugly if you don’t do the math, as it is easy to create a feature that’s severely undercosted if you’re playing with numbers that only come up 5-15% of the time.

For the next part, check out this sidebar from James Wyatt, in the 3rd Edition Rules Compendium:

EVOLUTION OF SPELL RESISTANCE

Back in the old days of the original Advanced Dungeons & Dragons game, monsters had magic resistance, expressed as a percentage chance that a spell cast by an 11th-level wizard would fail against the monster. That chance went up or down by 5% for each level of difference between the caster and 11th level—or it did, if you played strictly by the rules. I’m not sure I ever did.

One interesting aspect is that magic resistance was a percentage chance that meant a different thing for different levels of monsters. A low-level monster with 50% magic resistance was immune to the spells of a 1st-level caster, so 50% resistance was really bad for a low-level caster. A high-level monster with 50% magic resistance had no defense against a 21st-level caster, so the same percentage was really good for a high-level caster.

The rules of Second Edition simplified magic resistance into a straight percentage chance that the monster could ignore a spell. So 50% magic resistance negated half of all spells, without regard to the level of the spellcaster. It was just as good for a high-level monster as for a low-level monster. But if a high-level caster faced low-level monsters, his spells might still fail—this system didn’t respect the level of the caster at all. Spell resistance in 3e is a whole lot cleaner, while still having a lot in common with the First Edition system. If a CR 11 monster has SR 22, then a spell cast by an 11th-level wizard has a 50% chance of failing against that monster, because the caster needs to roll an 11 or better on 1d20 to affect the monster. But if the spellcaster is higher or lower level than the monster, that chance of failure goes up or down by 5%.

This system respects the caster’s level, like the First Edition system. But it gives you a consistent numerical scale, like the Second Edition system. It’s the best of both worlds.

Wait, what does spell resistance have to do with critical hits? Well, look at the emphasis on “clean scaling” and consistency across various levels.

I’m pointing this out because the crit confirmation roll is essentially the “hits the target by a margin of 5 or more” clause in the AD&D 2e system, except it scales with the characters involved. That is, the margin adjusts to whatever it is you need to hit the target in the first place, but you’re still avoiding that same situation they wanted to prevent in AD&D 2e where only a natural 20 will hit, and all of those hits are automatically crits.

Having tackled all of this so far, the next part of our discussion will have to delve into the issue of critical hits always doing more damage than normal hits. In video games, it’s easy to take this sort of thing for granted, but mostly because A. some if not most attacks are automated, B. you have a computer to do the math for you and C. you’re generally making a lot more attacks per combat.

In a tabletop environment, most combats are decided if not ended within a dozen rolls, so it can feel not-good to have a critical hit system where your crits are potentially just as strong as normal hits. Sure, the average might bear that out, but if you’re not seeing that average within a typical gaming session because combats, and therefore attack rolls, and therefore critical hits are infrequent, it can be difficult to tell yourself that it works out in the long-run.

So, going back to the AD&D critical hit system, suppose your normal attack is a 1d8+4. That has a maximum value of 12 damage. If you score a critical hit, double damage dice plus flat bonuses will give you a 56.25% chance of getting at least 13 damage.

With 3rd Edition critical hits, 2d8+8 has a 90.63% chance of dealing 13 damage or more. That’s much better! The trade-off though, is that between a narrower critical threat range and the need for crit confirmation rolls, the odds of actually scoring a critical hit are that much lower.

This brings us to 4th Edition, which further iterated on the critical hit mechanics:

1. When you roll a natural 20 on an attack, that is an automatic hit.

2. If 20+modifiers is not high enough to meet or beat the target’s defense, then it’s just a normal hit. For all practical purposes this should never happen.

3. If 20+modifiers is high enough to meet or beat the target’s defense, that is a critical hit. In effect, the roll that triggered the crit is also its own confirmation roll.

4. The damage of a critical hit is then defined as the maximum value of a normal hit, plus a rolled value (typically 1d6) per +1 enhancement bonus, which is generally accessible to players as early as level 1. 4th Edition crits also maximized the damage dice rolled from any other effect that would happen with a normal hit, such as Sneak Attack damage, whereas 3rd Edition specifically disallowed this.

This addressed the perceived issues with the 3rd Edition critical hit system: Critical hits would now always deal more damage than normal hits, and they would generally occur more frequently. There’s still that clause in there about how you don’t get an automatic crit on a 20 if you needed a 21 to hit, but that’s never really come up outside of edge cases where the party is fighting a Soldier that’s 3 levels above them while taking an attack penalty of some sort.

Finally, we get to 5th Edition:

1. A natural 20 is an automatic hit, and also an automatic crit. No confirmation rolls necessary.

2. A critical hit’s damage is defined as rolling damage dice twice, with flat bonuses left as-is.

This edition gets rid of the crit confirmation entirely, although at this point it was really more of a formality because of how the game as-written shouldn’t place players in situations where they’re fighting monsters that need such high rolls in order to be hit.

It does dial-back the damage rolling to AD&D standards, but it takes a page from 4th Edition where it allows additional damage dice from special abilities such as Sneak Attack to also be rolled twice.