The Four Core Mechanic Families
d20 + modifier vs DC (D&D, Pathfinder). d100 roll-under (Call of Cthulhu, Delta Green). Dice pool count successes (World of Darkness, Shadowrun). Dice pool roll-and-keep (L5R).
d20 + modifier vs DC: the most common mechanic. You roll a d20, add a modifier, and compare to a target number. This is D&D, Pathfinder, and most OSR games. The math is simple to explain but can produce swingy results because the d20 has a flat distribution (every number is equally likely).
d100 roll-under: you roll percentile dice and try to roll under your skill. Call of Cthulhu, Delta Green, and Warhammer Fantasy use this. It is more granular (1% increments) and easier to intuit (skill 60 = 60% chance), but math at the table is slower than d20.
Dice pool (count successes): you roll a handful of dice and count how many meet or exceed a threshold. World of Darkness (d10s, 8+ is success), Shadowrun (d6s, 5+). Dice pools produce bell curves naturally (more consistent results at higher pools) and allow partial success states (2 successes vs 5 successes).
Dice pool (roll and keep): you roll a number of dice but only keep some. Legend of the Five Rings (roll Skill+Attribute d10s, keep Attribute of them). This creates interesting tension between rolling more dice (higher ceiling) and keeping more dice (higher floor).
Damage Scaling: HP, Mega-Damage, and Wound Tracks
Escalating HP (D&D) creates zero-to-hero. Wound tracks (Savage Worlds) create gritty survival. Mega-damage (Rifts) is a design choice to make certain threats feel categorically different.
Most systems use one of three damage models. Escalating HP (D&D 5e, Pathfinder 2e): your capacity to take damage grows with level. This creates a clear power curve — the level 10 Fighter can survive falls that would kill a level 1 Fighter — but can feel unrealistic. Wound tracks (Savage Worlds, Fate, Year Zero Engine): you have a fixed number of wounds or conditions. Every hit matters regardless of level, which makes combat feel deadly but can make long campaigns feel punishing.
Mega-damage (Palladium/Rifts): a separate damage scale for vehicles, robots, and supernatural beings. 1 MD = 100 SDC (structural damage). A laser pistol deals 2d6 MD (200-1200 SDC equivalent). This system makes armored threats feel genuinely different from unarmored ones, but the math creates a two-tier combat system where unarmored characters are irrelevant against MD threats.