3 for 1 basis

3 for 1 basis or 2 for 1 basis refers to an ambigiously-worded rule appearing in the original 1974 edition of Dungeons & Dragons.

Origin

In Men & Magic (1974), each character class has a prime requisite, its most important of the six standard ability scores - strength for fighting men, intelligence for magic-users, and wisdom for clerics. If this is high enough, the character receives a percentage bonus to all experience gained.

However, some ambigiously-worded rules also explains that characters can "use" other ability scores in some way. For example:

Wisdom is the prime requisite for Clerics. It may be used on a 3 for 1 basis by fighters, and on a 2 for 1 basis by Magic-Users, in their respective prime requisite areas.

Possible interpretations

Points shift

A widely used understanding of this rule is that it allows players, at character creation, to subtract points from certain ability scores and add them to their prime requisite.

According to this interpretation, a fighting-man who has a Wisdom of 15 might decrease it by six points to 9, and in exchange, raise his Strength by two points. This appears to be supported by a note on page 11, following prime requisite:

Note: Average scores are 9-12. Units so indicated above maybe used to increase prime requisite total insofar as this does not bring that category below average, i.e. below a score of 9.

The descriptions of both Strength and Intelligence specify that the player may only add to their prime requisite for the purpose of gaining experience, not other benefits. There is some further ambiguity as to whether this limitation applies to Wisdom.

This interpretation is formally reflected in Moldvay's Dungeons & Dragons (1981), where Strength, Intelligence and Wisdom may be lowered on a 2 for 1 basis to raise a prime requisite. In that edition it applies for all purposes, not just experience.

Alternate interpretation

A less popular understanding of the rule is that it does not lower any ability score. Rather, a character simply adds one-third or one-half of certain ability scores to their prime requisite, for the purposes of determining whether they receive a bonus to gained experience.

For example, a fighting-man with a Strength of 14 and Wisdom of 12 could simply "use" three points of Wisdom to increase his Strength to 15 and narrowly qualify for the 10% XP bonus. The character does not actually lose Wisdom or gain Strength.

A counter-argument to this interpretation is that a fighter with 10 Strength and 10 Intelligence could use his Intelligence on a 2 for 1 basis and qualify for the 10% XP bonus despite being average.

Therefore, for this to make sense, it has to be assumed that the note on page 11 is supposed to mean you can't "use" the first nine points of an ability score. For example, a fighter with an Intelligence of 12 could use 2 points, but not 4 points, since that would "reduce" Intelligence below 9.

Official interpretation

Answering a question in an ENWorld forum thread, D&D creator Gary Gygax suggested that the less common second interpretation of the "3 for 1" rule is the correct one: players do not reduce an ability score, merely count part of it toward their prime requisite for the purpose of meeting the XP bonus threshold.

Doug: What the heck does that mean? That, for instance, clerics can add Wisdom + 1/3 of their Strength to determine their experience bonus?

Gary Gygax: You got it.

It seemed like a good idea at the time, this substitution enabling the 10% XP bonus to PCs with high abilities assocoated with but not their prime one, such as the cleric using 3 Str to equal 1 Wis.

DMs didn't think it was a particularly useful rule, so it was dropped.

— Gary Gygax Q&A, Part XIII, ENWorld Forums