I'm trying to determine which of the following is easier to understand:
#1 To make an attack, roll 1d20. Subtract the target's armor class from the roll. If your modified roll is greater than or equal to your attack throw value, you hit.
OR
#2 To make an attack, roll 1d20. If your roll is greater than or equal to your attack throw value, you hit. The target’s Armor Class is added to the attack throw value necessary to hit it.
OR
#3 To make an attack, roll 1d20. Apply the target’s Armor Class as a penalty to your die roll. If your modified roll is greater than or equal to your attack throw value, you hit.
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#2 is definitely clearest (most clear?). Except I think you have unclear antecedent: "The target's Armor Class is added to the attack throw value necessary to hit."
I've taken to explaining it like this: Roll 1d20 and subtract your attack throw; you hit any armor class up to the result.
Reading it cold, #3 was the only one I understood. I don't much care for the use of second person "you", though.
To make an attack, roll 1d20. Apply any situational modifiers, and see if the result beats your attack throw. If not, you miss. If so, your opponent's armor might still stop the blow. You only hit if your roll beats the throw by as much or more than the enemy's armor class.
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With the King characters this sequence didn't save much time, but at low levels it is very easy to say "well I needed a 14 or better and the roll isn't within a few points of that, I don't need to consider his AC or look for a bonus or two I might have missed."
Delta and others have pointed out that mental subtraction is harder than mental addition, and I do feel that the explanations that involve "minus" seem harder. Concealing it as "how much more than" is a cheap dodge in my text, but I think the step where you get to not do any mental math at all if the roll isn't close to the throw value makes up for a little time lost to subtraction; having the description highlight that step is important IMO.
"To make an attack, roll 1d20. Apply any situational modifiers, and see if the result beats your attack throw. If not, you miss."
This part, I think is right on, though perhaps you could simply say 'modifiers', then remind us AC is one of them:
"To make an attack, roll 1d20. Apply any modifiers (including your opponent's AC), and see if the result beats your attack throw. If not, you miss."
I think the 2 omitted sentences were somewhat confusing/strangely worded.
AC is positive. Most other modifiers will be +1 means I am more likely to hit, -1 less likely. AC has to be treated differently because it breaks the pattern.
I'd love to see your rewrite of that idea, though!
Since modifiers are always positive, and AC is positive, AC could modify your Attack Throw.
"To make an attack, roll 1d20 and apply any situational modifiers to the result. If this beats your attack throw, modified by your opponents AC, you hit."
One drawback of the all-positive description is that it breaks the way most games I run and/or play in report rolls: The players roll, and tell the judge what AC they would hit. To me, this is a streamlining conceit, as only one person at the table needs to know all the targets' ACs. I guess that's why I use the description I mentioned, which it occurs to me now is THAC0 + ACKS AC system.
The system in ACKS currently has gotten me into a habit of giving out the AC of the creature anyways.
"The goblin's AC is 3 - it has leather armor and is very agile."
Then, the player tells me whether they hit or not based on their attack throw.
I agree with Charlatan. (Yes, I do that a lot.)
In my experience players usually roll dice and call out "I hit AC 3" or the like, and the DM adjudicates. Phrasing that supports this would be my preference. Whether that works with positive AC is another question.
How about this:
To hit an unarmored target, a character must roll 1d20 and equal or exceed his attack throw value. If the target is armored, the character must equal or exceed his attack throw value by the target's armor class.
To make an attack, roll 1d20. Apply any modifiers, and see if the result beats your Attack Throw. If not, you miss. Certain modifiers (such as your opponent's AC) are added to your Attack Throw, making it more difficult to hit your foe, while others (such as the bonuses provided by a magic weapon or weapon focus) are added to the die roll, making it easier to hit.
And then perhaps something like "Individual descriptions will tell you what modifiers apply as well as what to apply them to."
Honestly, I'm not sure any abstract description of attack throws is going to be perfectly clear, so I'd think about going with concise and including an example.
I presented 5 different variations of the rules text to different casual gamers unfamiliar with ACKS. They unanimously picked the following text:
"To make an attack, roll 1d20. Subtract the target's armor class from the roll. If your modified roll is greater than or equal to your attack throw value, you hit."
It may be that in practice, mental addition is easier than easier mental subtraction, but in terms of writing a clear rule, expressing armor class as a subtractive penalty seems to carry the day.
I thought the advantage of this system was that you just added the target's AC to your hit number and that was your target number to roll. Seemed simple enough to me.
"To make an attack, add the targets armour class and any other modifiers to your attack throw value and then roll 1d20. If your roll is more than your modified attack throw value, you hit."
Hmm.. that's my third or fourth rewrite. I think I agree with Alex that while mental addition is easier, AC subtraction is easier to express when written down.
"To make an attack, roll 1d20. Subtract the target's armor class from the roll. If your modified roll is greater than or equal to your attack throw value, you hit."
I agree, this is definitely the most clear so far. A little subtraction isn't gonna kill anyone, especially if it makes the rule clear. People are still at their leisure to modify their way of looking at it based on their group's preference.
quick anser from me: #2