Source: Clerics Wear Ringmail: Dice Pool Roll Under

Rolling under your abilities.

One of the most controversial of ruling suggestions in B/X – nested snugly into the subsection, Dungeon Mastering as a Fine Art on page B60 under the moniker that There’s always a chance. This rule has become a default mechanism for some referees – it’s simplicity and quickness at the table while differentiating characters more or less naturally capable luring them in – and has become anathema for others: it’s emphasis on abilities defying the class-level focus of almost any other challenge in the game, it’s swingy potential for a well-endowed character to fail while an incapable character succeeds (the “I loosened it up for you” hypothesis) puts them off. However – in recent vintage – I’ve come across a variant which may help to alleviate some of the tensions – rolling under using a d6 dice pool.

My interpretation as follows:

Pooling Under your Abilities

When presented with a task – skill, talent, or otherwise – not covered by a rule but requiring a roll to account for potential of failure, the referee should, having considered the described manner of resolution, ask the player to roll a number of d6 proportional to the difficulty of the task compared to the efficacy of the proposed resolution mechanism:

• Easy tasks, or tasks well achieved, roll 1d6.
• Normal tasks, or perhaps easy tasks the method of resolution provided being questionable, roll 2d6.
• Challenging tasks, or tasks being approached poorly, roll 3d6.
• Difficult tasks, or tasks being approached entirely wrong, roll 5d6.
• Impossible tasks, or descriptive equivalent, roll 8d6,

If the result of the pool exceeds the ability score of the character rolling, the task fails. Otherwise, the task succeeds.

Optionally, to represent the off-chance of failure even when doing something simple (or, I Missed My Mouth With The Cup syndrome), use single-explosion dice. Thus, an Easy task has a likely result of 1-5, but an unlikely result of 7-12; or a Difficult task has an average result of 17 or 18, but a potential (and highly unlikely) result of as much as 60.