[RPG] What are the advantages to mixing roll-over and roll-under mechanics


Some role-playing game systems—like Dungeons & Dragon before its third edition—use a combination of roll-over and roll-under mechanics (“roll-over”: high rolls vs. target number are good; “roll-under”: low rolls vs. TN are good).

Does this mixture of mechanics have any design or gameplay benefits that make it superior to using a roll-over only mechanic or a roll-under only mechanic?

Best Answer

I have done some game-design work, mostly in D&D 3.5e-derived settings, which are roll-over. However, some roll-under ideas were considered—albeit briefly—for some of these mechanics, so I’ve given this some thought and discussed these with other designers. I cannot claim as much experience with this question, though, as someone who has done a lot of work with roll-over, roll-under, and mixed systems. But from my perspective,

There is no particular “advantage” to mixing the two; that is pure downside, because it makes the game less consistent and forces players to remember which rolls are which.

But picking one and enforcing consistency has its own downside: there can be advantages to one or the other in certain situations, but if you enforce consistency you cannot use the “better” approach when it would make sense. For examples:

  • Roll-under has the nice property for percentile rolls that your target number is also your chance of success: if you must roll a 20 or less on a d100, you have a 20% chance of succeeding. For roll-over, the same 20% chance would be a roll above 80—you have to do 100−x to determine your odds each time.

  • On the other hand, roll-over works much better for unbounded numbers: you can always increase a target number in roll-over, but decreasing a target number in roll-under is somewhat awkward when it gets negative.

In short, by not having the game consistently use one or the other, you are free to pick whichever is appropriate for a given roll, not being constrained by the game using the other type for everything else. The downside is, the freer you are with this—the more you use whatever roll type seems most appropriate for each roll, the harder it’s going to be for players to remember which rolls are which.

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