#### Voadam

##### Legend

Quick: what’s 17-12?

17 - 10?

17 - 8?

17 - 19?

congratulations, you just used THAC0 to work out the armour hit 4 times in quick succession. That’s all there is to it.

If you have a +1 to hit with dex this turns into two options

Take your +1 to hit and pre modify your THAC0 by subtracting the +1 from the THAC0 number to get the number you need to hit AC 0.

So

16-12

16-10

16-8

16-19

So it is still a one factor calculation.

Or you can add the +1 to the attack roll as a bonus to the second part to figure out the roll.

17-(12+1)

17-(10+1)

17-(8+1)

17-(19+1)

both work mathematically but the first is a little more straightforward on the fly.

A PC can reasonably choose whatever way they want and get to the same result.

However in B/X, BECMI, RC you also have situational things like your ranged attack probably being at short range for +1 on that specific attack.

So the two options then look like this:

16-(12+1)

16-(10+1)

16-(8+1)

16-(19+1)

17-(12+1+1)

17-(10+1+1)

17-(8+1+1)

17-(19+1+1)

You could alternatively add in the +1 to attack as an on the fly minus to the THAC0 part of the equation for more options

(16-(+1))-12

(16-(+1))-10

(16-(+1))-8

(16-(+1))-19

(17-(+1))-(12+1)

(17-(+1))-(10+1)

(17-(+1))-(8+1)

(17-(+1))-(19+1)

For ascending AC It becomes

12+3+1+1

10+3+1+1

8+3+1+1

19+3+1+1

Or if they combine their dex mod into their ranged attack on their sheet it becomes

12+4+1

10+4+1

8+4+1

19+4+1

In ascending AC the DM can ask what did you roll and what are your bonuses and all the factors are captured.

In THAC0 the DM can ask what did you roll and what is your THAC0 and that might or might not capture all the relevant modifiers depending on how the calculation is (validly) being done.