Here is a general ability check system suitable for use in AD&D games. It is quite general, and it fits with the mechanics already in use. The system is based on the mechanics used for the “open doors” strength check, and is extended to other abilities. To each ability score a success probability is assigned, based on a d6 roll, as follows:

Ability score Success range3-7 18-15 1-216-18/50 1-318/51-18/99 1-418/00 1-5

The percent score is obviously used for strength checks, but classes such as UA Paladins and Cavaliers have other exceptional ability scores, so the system can be applied to these classes, too.

Modifiers to the success range can be applied to accrue for different conditions, from extremely easy (+3) to extremely complex (-3), the default being for tasks of moderate complexity. As an example, forcing open a magically held door might warrant a –3 penalty, which only characters with 18/51 or greater strength can accomplish, with success ranges of 1 and 1-2 (for 18/00 strength). Note that this example would correspond to the values reported in the PHB.

Primary abilitiesThese are defined as the first three abilities