I feel like THAC0 gets a bad rap. It's the precursor to 3E's Base Attack Bonus, and came after 1E's attack matrices. It's actually super simple.
The important thing to remember is that it's the exact same math, but in reverse.
In 3E-5E, you roll d20 and add a bonus in order to beat the target's AC.
In 1E-2E, you roll d20 in order to beat THAC0 minus AC.
So instead of adding a number to your d20, you simply deduct a number from your THAC0. Same maths, just minus instead of plus.
If your THAC0 is 15, and your target's AC is 2, you need to roll 13. It's that simple! (Remember, lower AC was better).
I feel sorry for poor old THAC0. It has a bad reputation for being complex, when it's exactly as complex as the current method!
The important thing to remember is that it's the exact same math, but in reverse.
In 3E-5E, you roll d20 and add a bonus in order to beat the target's AC.
In 1E-2E, you roll d20 in order to beat THAC0 minus AC.
So instead of adding a number to your d20, you simply deduct a number from your THAC0. Same maths, just minus instead of plus.
If your THAC0 is 15, and your target's AC is 2, you need to roll 13. It's that simple! (Remember, lower AC was better).
I feel sorry for poor old THAC0. It has a bad reputation for being complex, when it's exactly as complex as the current method!
Last edited: