I have this idea in my head and no idea how to use it. But it's been bugging me for a long time, so I'm going to write it out and maybe one of my gentle readers can figure out how to make it work. The idea is about the ubiquitous "to-hit" roll. Combat is an abstract, moreso in some systems like D&D (Monsters & Manuals has a good post about that), but it always covers several different possibilities. My idea is to just break down the possibilities into a stack, and the to-hit roll then describes what happens in that stack. Not making any sense yet, bear with me:
The first, base part of the stack, is Dodge. Dodge is how hard you are to hit in the first place. This is a function of size, dexterity/nimbleness and awareness (hard to dodge something you can't see).
Then we have Block. Shields are the stereotypical defensive weapon, as they are mobile coverage, but weapons are also defensive. Parrying an attack with a sword means that attack didn't hit you, right? So block covers anything that will prevent an attack from landing on you. Note, this then does not count blocking with your forearm or other body part - since the attack is hitting you, just with possibly reduced damage (however a special ability/feat may allow you to use your innate block against weapons by saying you block the flat of the blade or the arm/limb that's attacking or somesuch).
Next part of the stack is Coverage. This is a function of how many hit locations/body parts you have covered. Full plate covers a lot of your body (but still has a few holes/vulnerabilities) while a mail shirt only covers some - but a spherical force-field covers everything.
Sort of combined with Coverage is Openness. Basically, what of you isn't covered. This is a separate part of the stack for reasons I'll make clear in a moment. It is a split between the two, so there is a total of, say, 50 points to represent your size and body (for medium creatures, possibly modified for larger/smaller?). Wearing armor that covers half your hit locations means you have 25 points of Coverage and 25 points of Openness.
Lastly is Vitals, which is a modifier of Openness and based on your creature type. People are funny, we can actually take a lot of abuse, but we have some very soft and squishy spots that can kill instantly (or nearly so) if damaged. So this is the 'critical hit locations.' It is a fraction of your Openness (say, 10%), representing the vulnerable sports on your body that are not protected by your armor/defenses. However, some creatures like undead and constructs do not have any squishy bits, so they do not have a Vitals score.
Now, my crazy idea is this- what if we give a score to each one of these, and then stack them on top of each other, and then roll to-hit and see where in the stack we land, which will tell us what kind of hit we made?
Example, say you target is a goblin. He's small and fast, so he's got a Dodge of 40. He has a small shield and short sword, but being smaller than human their defense is reduced, so they total a Block of 10. He's got patchwork armor over half his body (say, chest and both arms), making his Coverage 25 and Openness 25. And being a living creature he's got squishy bits for a Vitals of 3. If we stack these from Dodge up we'd have:
Dodge 1 - 40
Block 41 - 50
Coverage 51 - 75
Openness 76 - 100
So, if we rolled percentile (1-100) we could see exactly what happened. A roll of 23 is a total miss, the goblin danced out of the way. A roll of 37 means the goblin deflected the attack with his shield, still a miss but maybe costs him 1hp for the effort of the block (or perhaps the shield takes the damage and could possibly break?). A roll of 60 is a solid hit on his armor, so subtract the armor's damage reduction from the attack. A roll of 88 is a hit on the unarmored part, so you cut the goblin's leg and it takes full damage. Hitting the vitals, rolling above 100 would require some type of ability or "aim" action that gave you a bonus chance to hit - and could mean anything from blindness to instant death.
What makes the idea go crazy, and turns my brain in knots, is what happens if you can alter the stack. So, say you have an armor-piercing weapon, it totally ignores armor (or, at least the type of armor the goblin is wearing). Now the stack's bottom is the same: 1-40 Dodge, 41-50 Block; while the top changes to Openness 51-100, Vitals 101-103. Say you feint, removing the ability of the goblin to block- 1-40 Dodge, 41-65 Coverage, 66-90 Openness, 91-93 Vitals. You know, actually looking at this on paper I realize you don't need a Vitals score, just to say weather the target has Vitals or not since it is always the top-most range of possibilities. (though, it might be interesting to say that going over the vitals means a miss - you're trying so hard to hit a small target that you completely blow it; but that seems like it would make getting a critical hit harder than necessary, hmmm...) Or if you entangle/tie up the goblin so he can't dodge or block: 1-25 Coverage, 26-50 Openness, 51+ Vitals (which works for me, there are stories of headsmen who needed more than one swing to execute someone). Attacking from the sides or rear might negate the Block part of the stack too.
Let's add another layer of complexity- what if you could modify the roll to hit as well? So, say that everybody rolls d20s to hit, the number of dice being determined by class. Wizards roll 2d20, Thieves roll 3d20, Clerics roll 4d20 and Fighters roll 5d20. We'll let strength add to the roll, so a strength of 10 is a +10 to the dice roll. So the Str 10 wizard has a pretty low chance of hitting the goblin, and will get blocked even on a good roll - the goblin's just too fast. Thieves might get the armor, clerics might get an open shot but only the fighters have a chance at the vitals. Again though, if you can aim, or cast a spell boosting your to-hit or something you can make the odds better (or sneak attack, can't dodge/block what you can't see). And against a slow moving zombie everybody has a better chance to hit (though no Vitals since it's undead - or maybe it does have Vitals if it's a shoot-em-in-the-head type zombie).
I just love this idea because it breaks things down explicitly, none of the annoying "what does a hit point or attack roll really mean" abstraction in a game like D&D. But I hate this idea because it means a whole lot of extra calculations and possibly dice-rolling that seem like they would negate the benefits. I don't know if I'll ever think of something to make this work. Actually, I've been thinking of going the opposite direction, making combat more abstracted than even D&D by having the character roll both offense and defense for each round (which I realize says nothing, I'll maybe post later about some of my combat ideas and the great bloggers who inspired them). Anyways, now that I've writen this out hopefully it will stop scratching at the back of my head and leave me alone. I don't think I'll ever use it, but if you can figure out a way to make it work - go for it. And as always, comments are welcome.
PS- I think the impulse to post this came from reading B/X Blackrazor, he has a recent series on "fighter love" that is worth reading: http://bxblackrazor.blogspot.com/2013/08/fighter-love-2-combat-stunts.html (for part 2, also look for part 3). And has a cool combat idea of skipping the to-hit roll and just rolling damage to speed things up: http://bxblackrazor.blogspot.com/2010/05/radically-faster-combat-auto-hits.html