The d20 System
Core resolution mechanic: roll d20 + modifier vs DC, with advantage/disadvantage, contest, passive check, and group check rules.
Nearly every meaningful action in the game revolves around rolling a twenty-sided die. When a character attempts something with a meaningful chance of failure, the DM calls for a d20 roll, adds the relevant modifier, and compares the total to a target number.
Ability Checks
Roll d20 + ability modifier (+ proficiency bonus if proficient in the relevant skill or tool). The DM sets a Difficulty Class (DC):
- Very Easy: DC 5
- Easy: DC 10
- Medium: DC 15
- Hard: DC 20
- Very Hard: DC 25
- Nearly Impossible: DC 30
Meet or exceed the DC to succeed.
Advantage and Disadvantage
When circumstances grant advantage, roll two d20s and take the higher result. When disadvantage applies, roll two d20s and take the lower. Multiple sources of advantage or disadvantage do not stack — you either have it or you don't. If you have both advantage and disadvantage simultaneously (from any number of sources), they cancel out and you roll normally.
Contest Rolls
When two creatures act in direct opposition (e.g., one tries to shove while another resists, or one hides while another searches), both roll d20 + relevant modifiers. The higher total wins. On a tie, the situation remains unchanged — the initiator fails to alter the status quo.
Passive Checks
A passive check represents an average result without rolling: 10 + all applicable modifiers. The most common passive check is passive Wisdom (Perception), used to detect hidden creatures and traps without actively searching. Advantage on the check grants +5 to the passive score; disadvantage imposes -5.
Group Checks
When the whole party attempts the same thing (e.g., sneaking past guards), the DM may call for a group check. Everyone rolls, and if at least half the group succeeds, the group as a whole succeeds.