+1d and -1d

Sometimes a special ability or spell tells you that you have +1d or -1d on a skill check, or an attack roll. Each instance of this modifies the number of d20s in your dice pool by 1. If the total number of dice in your pool is positive, use the highest of the rolls. If the total number of dice in your pool is less than 1, use the lower roll. For example, if you have 0 dice in your pool and roll a 17 and a 5, you use the 5. If you have 2 dice in your pool and roll those numbers, you use the 17.

If you have +1d on a damage roll, you similarly roll all the dice twice and take the better result.

If multiple situations affect a roll and each one grants +1d or imposes -1d, you roll more than one additional d20. For example, if two favourable situations grant +1d, then you roll 2 additional d20 and take the best of all 3. Similarly, if 2 unfavourable situations impose -1d, then you roll 2 additional d20 and take the worst of all 3.

If circumstances cause a roll to have both +1d and -1d, you are considered to have neither of them, and you roll one d20. This is not true if multiple circumstances impose -1d and grants +1d in uneven amounts. In such a situation, one case of +1d cancels one case of -1d, and you are affected by whichever condition there is more of.

When you have +1d or -1d and something in the game, such as the halfling’s Lucky trait, lets you reroll the d20, you can reroll only one of the dice. You choose which one. For example, if a halfling has +1d on an skill check and rolls a 1 and a 13, the halfling could use the Lucky trait to reroll the 1.

You usually gain +1d or -1d through the use of special abilities, actions, or spells. Inspiration can also give a character +1d on checks related to the character’s personality, ideals, or bonds. The GM can also decide that circumstances influence a roll in one direction or the other and grant +1d or impose -1d as a result.

Proficiency

Characters can also have +1d due to proficiency on attack rolls, skills, tools and defences. Monsters can also have this proficiency die, which is incorporated in their stat blocks. The bonus die is used in the rules on skill checks, attack rolls and defences.

Your proficiency die can’t be added to a single die roll or other number more than once. For example, if two different rules say that you can roll your proficiency bonus die when making a Wisdom check, you nevertheless add only one die when you make the check.

Expertise

You can have expertise with a weapon, skill, tool, or defence that they are already proficient in. A character with expertise benefits from an additional +1d on their d20 roll. Additionally, passive scores that benefit from expertise increase by an additional +2, for a total of +6 together with the proficiency bonus.

Mastery

You can be a master with a weapon, skill, tool, or defence that you are already an expert at. A character with mastery benefits from a third +1d on their d20 roll. Additionally, passive scores that benefit from mastery increase by an additional +1, for a total of +7 together with the proficiency and expertise bonuses.