Ability and Proficiency Checks

Ability and proficiency checks are successful by rolling 3d6 under the ability check score. A modifier for difficulty may be applied to the ability check score by the DM. Difficulty modifiers range from -10 (Near Impossible) to +10 (Certain Success). A 3 always succeeds and an 18 always fails. The ability check score is generated by dividing the ability score by 3 dropping any remainder and adding the integer to 9. This will create a check score between 10 and 15. The ability check score is modified by proficiency rank with a skill, the proficiency modifier, and task difficulty.
Ability and Proficiency Check Difficulty Chart
Example #1:
Starken, a thief with 2 ranks of proficiency in disguise, attempts to disguise himself as a city guard to infiltrate the inner gates of the city. This situation calls for a disguise check, using the charisma ability. The disguise proficiency has a check modifier of -1. Because Starken is trained in disguise and the guards are not alerted or suspicious, the DM rules the task is average difficulty with a 0 modifier. Starken's charisma score is 13. His ability check score is 9 + (13/3) +2(proficiency ranks) -1(proficiency modifier) + 0 (difficulty modifier) = 14. Starken will be successful if he rolls under this check score.
Example #2:
Dorn, a fighter with 0 ranks of proficiency in disguise, attempts to disguise himself as a city guard and follow Starken into the inner gates of the city. As with Starken, the situation calls for a disguise check using the charisma ability. The DM rules the attempt by Dorn is unfavorable. Dorn has no training in disguise and the situation is somewhat stressful as a result. Dorn's ability check score is 9 + (11/3) + 0 (proficiency ranks) - 1(proficiency modifier) - 1 (difficulty modifier) = 10. Dorn must roll under a 10 on 3d6 for the disguise to be successful.
The ability check score is figured before play starts by dividing the ability scores by 3 and dropping all remainders, and adding the result to 9. Dorn with an 18 strength has a Strength check score of 15 (18/3 + 9). Players are responsible for adding their non-weapon proficiency ranks to an ability check that a proficiency skill impacts.