Opposed dice rolls
Using opposed dice rolls as a resolution mechanism in tabletop games
Last week we looked at losing on purpose with the Loki strategy in Blood Rage, and turn order systems the week before that.
This week we look at a more general concept: opposed rolls!
Opposed rolls vs. target numbers
If you play tabletop roleplaying games, chances are you are very familiar with both opposed dice rolls and rolling versus a target number.
Here are just a few examples:
Target numbers
In many target number systems, the player rolls a single die (e.g. d20), adds or subtracts any modifiers, and compares versus a number. If the result is equal to or higher than the target, it’s considered a success.
MÖRK BORG provides a simple example of rolling versus a target number:
Each character has abilities (i.e. agility, presence, strength, toughness) with an associated ability score between -3 and +3.
When attempting an action (e.g. jumping across a hazard) the player performs a Test by rolling 1d20 plus their relevant ability score.
The result is compared against Difficulty Rating (DR), usually between 6 and 18. A DR of 6 is extremely easy, while a DR of 16 is really hard.
Results equal to or greater than the DR means the attempt was successful.
Other systems like Emberwind RPG use a similar system, but you need to roll under (less than) the character’s relevant skill. The higher the skill, the greater the chance of success.
Variants exist where you roll a pool of dice, and compare the highest or lowest value vs. the target. This allows modification to the chance of success without having to add or subtract modifiers.
Opposed rolls
An alternative to rolling versus a fixed target number is an opposed roll. Two (or more) dice are rolled, compared, and (usually) the highest result wins.
The classic example of opposed rolls is in combat systems. Both sides roll a single die (e.g. d6), and the side with the higher result wins. A mechanically similar system could use a deck of cards (e.g. the War card game).
In tabletop roleplaying games, opposed rolls are often used when two parties (e.g. characters, monsters, etc.) are in direct conflict with each other. These are called Contests in the Dungeons & Dragons SRD:
Both participants in a contest make ability checks appropriate to their efforts. They apply all appropriate bonuses and penalties, but instead of comparing the total to a DC, they compare the totals of their two checks. The participant with the higher check total wins the contest. That character or monster either succeeds at the action or prevents the other one from succeeding.
While most action rolls are versus target numbers (higher is better) in Blades in the Dark, it gives the option of using opposed dice in fortune rolls:
If no trait applies, roll 1d for sheer luck or create a dice pool (from one to four) based on the situation at hand. If two parties are directly opposed, make a fortune roll for each side to see how they do, then assess the outcome of the situation by comparing their performance levels.
Shadowrun handles opposed rolls using a variable size dice pool based on skills and attributes. Instead of adding the dice values, “hits” are counted. Both sides then compare their number of hits and the higher wins. Ties are either considered a stalemate or given to the defender.
It’s been a few weeks since we used a Python simulation, so let’s take a look!
Simulating opposed rolls
The above chart show the outcomes of opposed rolls between Player A and Player B for various types of dice (i.e. d4, d6, d10, d12, d20) where Player A has a simple, positive modifier applied ranging from +0 to +3. Player B has no modifier.
Each condition was simulated in Python 100,000 times and then expressed as a percentage. For example, in an opposed roll with d20+0, both Players A and B win about 47.5% each and tie 5% of the time.
A couple things are notable:
The lower the die type, the higher the impact of modifiers. This makes sense because a +1 modifier is a higher percent of the total range in a d4 than it is in a d20.
Ties are more likely with lower die types (i.e. d4) and with lower modifiers. With no modifier (+0) there’s an equal chance of success, so ties are very important. More on this below.
Somewhere around a modifier of +3, failure is extremely unlikely. It depends on the die type, but you can see this in the d4+3 roll above.
Modifier complexity
One thing I always think about is how mechanisms might be reduced and still have the same outcome.
An opposed roll where both parties have the same modifier has the same outcome as an opposed roll where both parties have no modifier. For example, if both sides roll 1d6+1 and compare, that’s the same as both sides rolling 1d6+0. This means that if both sides theoretically had many modifiers (1d6 +2 -1 +4) but the same sum, all that adding and subtracting didn’t actually change the outcome!
While some players can quickly do arithmetic in their head, others (like me!) struggle to add and subtract multiple modifiers. This is even more complicated when modifiers are applied to the opposing roll and not just your own (e.g. “You get -1 to hit me because I’m behind cover!”).
It’s important to consider the mechanical burden when designing modifier systems, or it might slow down the game and/or pull people out of the experience.
Ties in opposed rolls
With ties in opposed rolls possible about 5 - 25% of the time, it’s important how they are handled.
This can be resolved in many ways:
Attacker or initiator of the action always wins
Defender always wins
The party with the higher relevant stat, skill, or modifier wins
The higher rolled (unmodified) value wins
In the event of a tie in a D&D contest:
If the contest results in a tie, the situation remains the same as it was before the contest. Thus, one contestant might win the contest by default. If two characters tie in a contest to snatch a ring off the floor, neither character grabs it. In a contest between a monster trying to open a door and an adventurer trying to keep the door closed, a tie means that the door remains shut.
This is similar to the “stalemate or defender wins” used in Shadowrun.
Thematically, this seems to make sense in the examples given, and also gives advantage to one side (e.g. the door example above) when appropriate.
Opposed rolls in Root
Cole Wehrle’s Root (2018) does something interesting with opposed rolls when battles occur.
Players don’t “own” one of the two opposed dice that are being rolled. Instead, when a battle happens, two custom d12 dice are rolled each having only 0, 1, 2, and 3 values on the faces:
The attacker deals hits equal to the higher roll, and the defender deals hits equal to the lower roll. However, each player can roll no more hits than their number of warriors in the clearing of battle.
This is ultimately an opposed roll, but one in which the attacker always has an advantage by taking the higher roll.
It also handles ties in an interesting way! Players can strengthen their position with more units. If you have three cats and the opponent has two birds, and both dice are 2s you each lose a warrior. You still end as the victor, however, with one cat remaining!
Dice add chance, but players still have a large amount of decision and agency in determining the outcome.
It also allows for specific factions to bend the rules. For example, the Woodland Alliance (“experts in guerrilla war”) use the higher roll and the attacker uses the lower roll.
Opposed rolls in Oath
Oath: Chronicles of Empire and Exile (2021), also by Cole Wehrle, ramps up the complexity of opposed roll battles compared to Root.
Campaigns (as battles are called in Oath) use an opposed dice pool system with custom dice:
The attacker collects dice based on how many warbands (units) they have
The defender collects dice based on the locations being attacked
The defender rolls and adds up the number of shields (including x2 modifiers) plus the number of warbands they have at their sites. They might also add the warbands on their player board.
The attacker rolls their pool and adds up their swords: one for every filled sword, and one for every pair of hollow swords. They may also sacrifice warbands from their force to increase their result.
Both sides compare, and the higher result wins. The defender wins on ties.
My favorite part of this system is the ability for the attacker to sacrifice additional warbands to ensure a victory. It adds a tough decision based on how badly you want (or need) to win that Campaign.
Conclusion
Some things to think about:
Think about how the mechanism fits the context of the game. Opposed rolls might fit well when there are two parties in direct opposition, while target numbers might fit better for a player versus the environment (or themselves).
Consider each mechanism’s burden on the player. Each modifier may seem simple, but repeated addition/subtraction can quickly become taxing on players. Opposed rolls can sometimes (not always) take longer than target rolls. Playtesting can help identify any potential issues.
There are countless ways to modify opposed rolls to fit your game. You can use different dice for each side (e.g. d10 vs. d12), exploding dice, or dice pools. Other resources (e.g. Oath’s warbands) can be used to sway the result. There are far more options than just rolling and adding the totals.
Have you played Oath? What are your favorite opposed dice roll systems? Do you prefer straight totals and modifiers, or dice pools and hits/successes?
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See you next week!
— E.P. 💀
It's always important to get back to the basics. I sometimes forget about the different ways that rolls can be made. As an amateur TRPG content creator, I need to expand my knowledge. Thanks for this!
Quietly adding this particular post to my must-reads for game design. I would buy the Skeleton Code Machine posts compiled into a zine, I think.