On d6 Ability Checks

Abstract

Roll-under-ability checks using d6 dice represent a historically significant but undocumented D&D mechanic. While not officially written into OD&D or AD&D rulebooks, Gygax and other early referees commonly used this system for ability-based resolution checks. This article explores the mechanic's history, probability, and design implications.

Historical Context

Undocumented Mechanic

• Not written into OD&D or AD&D core rulebooks
• Gygax and Kuntz reported using this at their tables
• Appears in WG4 (The Forgotten Temple of Tharizdun) module as a roll-under-Dexterity-on-4d6 check
• Later officially suggested as optional rule in 1981 Moldvay Basic D&D

Design Question

The absence from official rulebooks raises the question: Was this so fundamental that Gygax simply didn't document it, or was there another reason?

Mechanic Explanation

Players attempting an ability check roll Nd6 and compare the total to their ability score. Success occurs if the result is equal to or below the ability score. The number of dice increases with difficulty:

• 3d6: Standard check (matches ability score generation)
• 4d6: Harder check
• 5d6: Very difficult check

Success Probabilities

Actual success rates vary dramatically by ability score and dice count. Notable findings:

Very Low Scores (3-6)

• vs. 3d6: Less than 10%
• vs. 4d6: Less than 10%
• vs. 5d6: Less than 10%

Design Implications

Advantages

• Theoretical elegance of uniform roll-under methodology across all checks
• Uses familiar d6 dice already used for ability generation
• Simple difficulty adjustment through die count (3d6, 4d6, 5d6)
• Fits naturally into OSR sensibilities

Disadvantages

• Makes ability scores critically important (contrary to OD&D philosophy, where low scores had minimal impact)
• Success rates become absurdly low for poor ability scores, raising the question: "Why roll at all?"
• Creates situations where rolling becomes merely theatric rather than mechanically meaningful

Player Agency and Character Concept

One of the core tensions: OD&D philosophy allows playing legitimate characters with unimpressive ability scores because they make little gameplay difference. Roll-under-d6 ability checks threatens this by making abilities hyper-relevant.

Practical Decision

One workable compromise is to use Nd6 roll-under as a save system rather than as a universal ability check mechanic. This preserves the gygaxian lineage while limiting the "low score viability" problem to high-stakes save moments rather than every action:

• 3d6 standard saves, 4d6 hard, 5d6 very difficult.
• Each ability governs a threat domain (CON = poison, DEX = avoidable harm, etc.).
• General skill adjudication uses a separate resolution framework (TBD).

Design Considerations for Modern Games

Should a game adopt this mechanic? Considerations include:

• Ability importance: Are you comfortable with ability scores dominating play?
• Low score viability: Will characters with poor abilities feel punished?
• Mechanical meaning: Is every resolution worth rolling for, or should some be automatic failures?

Mechanics Discussion

Whether this mechanic should be standard in a minimalist D&D system depends on one's philosophy about ability scores and player agency. It pairs well with low-powered, lethal play where abilities matter greatly, but conflicts with the OD&D approach of broad viability.

OD&D Chance-in-6 Ability Check Table

A later analysis (solodungeoncrawler, 2024) argues that the most internally consistent OD&D ability check method uses a chance-in-6 keyed directly to score ranges — matching the d6 probability used by OD&D for doors, traps, secret doors, and similar standard tasks (baseline 1-in-6 or 2-in-6).

The critique of roll-under d20: an average score of 10–11 yields a 45–50% chance of success, which is far above the standard 33% (2-in-6) challenge the game uses elsewhere. A chance-in-6 approach keeps ability checks calibrated to OD&D's baseline odds.

The Table

Recommended Ability Domains

Design Rationale

• Keeps ability checks probabilistically aligned with OD&D's standard task resolution (the d6)
• Score bands (low / average / high) mirror OD&D's own ability score framing in Men & Magic
• Directly useful for solo play where a consistent adjudication method prevents "winging it"
• Historically grounded in Wesley D. Ives's 1976 Dragon #1 framework (though Ives's version was more complex)

See Also

• Minimum Viable D&D - Broader discussion of core D&D mechanics
• The OD&D Engine - Core systems that omit ability checks
• State of the Art in OSR Rules Design - Modern mechanics innovations
• B/X Resolution Fragmentation - Analysis of fragmented checks and proposed solutions
• Solo Trap and Listen Detection - Solo procedure using level + INT bonus on d10

Sources

• https://solodungeoncrawler.blogspot.com/2024/08/od-ability-checks-why-rolling-under-is.html