(Yet another) critical hit system for B/X, AD&D etc.

 It is quite simple:

A) Natural 20 means maximum damage.

B) Beat AC by 10 or more means double damage.

This has several advantages.

"A" gives you a quick, optimal result that is still within the expected boundaries. You can deal maximum damage at any time, but a natural 20 guarantees it. No "whiff factor". The average damage is not significantly impacted.

And "B" gives you:

- The fighter to get a small boost in damage, especially against weak foes, which is good.

- Armor becomes more important - going unarmored is now a terrible idea.

- The thief gains more damage with backstabbing. This is good for B/X but maybe unnecessary for AD&D. OTOH the B/X thief becomes a bit more frail due to light armor and low HP.

Both give more importance to strength bonuses and magic weapons, and even make two-handed 1d10 weapons a bit stronger (although a shield also becomes more important).

But what if both happen at the same time?

There are several solutions.

- Double maximum damage. This would occasionally allow a B/X fighter that usually deals 1d8+2 damage to deal 20 damage with a single blow.

- Double damage, but ONE of the dice is automatically maximum damage (i.e., 1d8+3 becomes 8+1d8+6). I like this solution because the maximum damage is still impressive, but the average is a bit lower.

- Double damage plus another attack. I like this one because it gives the fighter some cleaving.

A caveat: monsters will get more dangerous too.

This is not a HUGE problem IMO; I like dangerous monsters, and with multiple attacks monsters are likely to spread the damage a bit. But it is something to keep in mind.