I can't remember how cover and concealment worked in all the various RPGs I've owned and/or played. This is what I'm gonna use until something better comes along. It's simple, consistent, and flexible covering a broad range of situations/needs thus reducing the number of rules I need to remember.
Cover is hard (it blocks or deflects) such as a mantlet or pile of fallen enemies. Concealment is soft (it obscures or conceals) such as drapes, vegetation, mist, and darkness. Both are quantified as a percentage from 0%-100% in 10% increments and use the same resolution mechanic. When rolling to hit against a target with a c&c > 0% an additional d10 is rolled. If that d10 roll is > than the c&c (converted to an integer from 1-10) then the attack is resolved as normal. Otherwise the attack missed original target regardless of tohit. In the case of cover the attack has hit it instead (which may or may not be important) and for concealment the attack has gone short/long/wild as determined by DM if necessary. Roll both dice together to speed play. Ex A poor goblin guard picking his nose behind a low log wall (50% cover) is being charged by Mog. On his way there he decides to introduce the goblin to his handaxe. Rolling a d20(18) whoohoo! and d10(5) curses Mog's axe buries it self in log inches from the goblin.
Cover & concealment rarely stacks. Ex: A sneaky no good elf in near total darkness (80% concealment) and behind a low wall (50% cover) would have 80% (not 130%) chance of being missed.
The max cover one can have and still make a melee attack is 70%. The max cover one can have and still make a ranged attack is 90%. There are no maximums for concealment. Remember that a round is 6, 10, or 60 seconds long and you should consider the average cover and/or concealment during that time. Ex: ducking a head round a corner should normally still be 100% cover. Silly elf prancing through the alternatively sunlight and shadowed branches should have half the shadowed (30% concealment), round down to 10% cause, it's an elf.
Concealment generally applies to all attacks, melee and ranged. Cover generally applies only to ranged attacks. But the DM may apply it in various situations in which it makes sense. Ex: attacking over a low wall, trying to attack the 2nd rank of a phalanx (1st rank provides cover for 2nd). Note melee cover is normally bidirectional.
Blindness is handled in terms of concealment. The newly blind face 90% concealment when attacking others. Reduced to 50% for those who have been blind long enough that their other sense have compensated. Of course any percentage of "blindness" maybe applied to represent various situations. Sand in face, 30% blind for a few rounds. Shooting into dazzling light, 50% blind.
Concealment can also be used for hiding/detection. Ex: It's not unreasonable to reason that Gilbert who is hiding in a bush (80% concealment) has an 80% chance to be spotted by the orc patrol stomping by.
Optional Rule: Magical/masterwork weapons add their to-hit bonus to the d10 roll.
Optional Rule: All manner of situations, as determined by the DM, may provide a bonus to the d10 roll. Ex: attacker's keen hearing
Creatures, Cover, & Concealment
Creatures generally provide cover vs ranged attacks. The amount depends on the size of the target, size of the creature providing cover, and situation. As a general rule similar sized creatures will provide 50% cover. Ex: mage lurking behind a meat-shield/fighter friend, spearman behind shield bearer. If a creature successfully provides cover it is subject to the attack using the original tohit roll. No new rolls are made.
Missile fire into melee is generally a crap shoot. The exact details are left to the DM to determine based on the situation. Ex: An archer looses one at the orc on the far side of and engaged in melee with his two fighter friends. Everyone being roughly the same size the DM gives the orc 60% cover, 30% from each of the two fighters. In this case on a d10 role of 1-3 fighter on left, 4-6 fighter on right, 7-10 the orc would be subject to the archers attack. A gnome might be no cover at all to a giant.
Shields and Cover
Shields provide cover vs ranged attacks. This replaces the shield's AC bonus, is effective vs only one "facing" per round, can not be used in melee (unless you're willing to give melee attackers big tohit bonus for not actively defending against them), requires awareness of ranged attack and active engagement of shield vs ranged attacks. You may move normal combat speed and use a shield to defend against attacks from any one facing. With faster movement such as a charge you may use a shield to defend against attacks only from the direction of movement (i.e. straight ahead). Ex: sneaky thief fires a sneakily poisoned quarrel at a guard leaning on his tower shield whose total cover is 0%. Note: you only have to be aware of the attack not be able to see your attacker. Ex: Seeing a volley of arrows fly over the castle wall.
10% improvised, untrained use of shield, weapon
20% buckler, special parry type weapon
50% large round/kite
70% tower (you could cower behind and get 90-100%)
Optional Rule: Magical/masterwork shields add their bonus x 10% to the amount of cover they provide.
Optional Rule: Small or large shield strapped to back provides automatic (i.e. no need for awareness or active engagement) 20% cover to attacks from rear facing.
Could use percentile dice or a d20 to have more granularity but why bother? Not a simulation. Two d20's would get confused. Thinking in nice round percentages or 0-10 is fast and simple (at least for me). Less is often more.
Cover and Concealment, a straight up percentage, ignores the skill of the attacker and the defenses(AC) of the defender. This provides tactical options to the weak (low level) or lightly armored when attacking a superior foe. This could aide the smart party at low level and provide new challenges (as defenders) to them at higher level.
Makes shields fairly effective against ranged attacks. Which is how it should be.* Although, it assumes you (and the player) take facing into account. Tactics and planning rise in importance. Ex: Mog has been flanked by a trio of crossboworcs on either side. Oh noes! He can only get the 70% protection of his tower shield against one group and perhaps neither if he's engaged in melee or isn't facing towards them for what ever reason.
The one issue I have with shields as cover (which I realized while writing this blog) is that an expert (high tohit bonus) has the same chance to get "blocked" by a shield as anyone. I'm not sure this is how it should be. Caused me abandon replacing shields AC bonus entirely and using the cover mechanics for melee as well. It is just too powerful. It verily irks me that there are two values for shields, that shield users have two AC's (melee/ranged). It makes what was an elegant system into a mess. So, much so I will have to drop it if I find no solution.
*I believe shields are highly undervalued by the mechanics of D&D (and have game-play** reasons for bothering to rectify this inaccuracy). The difference in ability of a naked man and a naked man with a shield to defend themselves is pronounced. When platemail protects against a blow it is still hit, it still hurts, it just wasn't lethal. When a shield blocks/deflects(which is more common btw) a blow it never hit! Shields rock!
**Better shields support several of my goals. Fighter options: using a shield or having off-hand attack is more of decision. Fighter power: ability to use shield is large benefit. Fighter options/power/weapon choice: Certain weapons like morning stars would negate shields cover partially or totally. Tactics, not tactical: Various tactics matter and smart players can gain advantage without having to battlemat it.
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