Each and every character has six principal characteristics, the character's
abilities. These abilities are strength, intelligence, wisdom, dexterity,
constitution, and charisma. (See also APPENDIX I , Psionic Ability.) The
range of these abilities is <generally> between 3 and 18.
The premise of the game is that each player character is above average
- at least in some respects -
and has superior potential.

Furthermore, it is usually essential to the
character's survival to be exceptional (with a rating of 15 or above) in no
fewer than two ability characteristics. Each ability score is determined by
random number generation. <when the scores are raised (before/after modifiers), and which scores (highest/lowest) are raised, makes a difference>

The referee has several methods of how this random number generation should be accomplished suggested to him or her in the DMG. <as well, Method V in UA>
The DM will inform you as to which method you may use to determine your character's abilities.

The principal abilities are detailed as follows:

Strength, Intelligence, Wisdom, Dexterity, Constitution, Charisma, Comeliness, Perception.

Q: In the old Giants in the Earth column, some of the heroes
were given a percentile score for every ability of 18, even those
characteristics like intelligence and wisdom that have no percentile
score given to them in the AD&D rules. Why was this done?
(SA53.12)

A: Originally such 18-percentile scores were given as a basis for
comparison; so many of the heroes and heroines in that column
had ability scores of 18 that it was thought there should be some
way to further differentiate between (for instance) two characters
who both have 18 intelligence. Such percentile values can be easily
disregarded, since they have no function in the AD&D rules.
(76.64)
 

A: Officially, none. All character classes are
assumed to exercise to a degree appropriate
to their professions, as a normal part
of training for advancement. The less physical
classes fill in their nonadventuring
time with study and mental
exercises.
(144.7)
 

A. No. The minimum scores required for
a character to become a paladin,
wizard or whatever apply only
when the character is created, not
once the character has started adventuring.
A paladin is particularly
vulnerable to changes in class -- if
the character's behavior strays from
LG, then paladinhood becomes a
normal fighter.
(Imagine #15)
 

the ability scores for PCs are requirements to enter the class and to progress within it thereafter.
In the case you cite, the PC would no longer be able to gail levels,
but he would remain a ranger of whatever level he had attained before dropping a point of Constitution so as to be inelligible for continued advancement.

Cheers,
Gary
 

It is if the PC is a worthy one from a player that simply had a run of bad luck. Also, keeping the HD gain to a d8 of the Ranger Class is a good comprimise in the bargain. After all, a Ranger is a fighter of sorts, so allowing progress thus is logical. I must say that if the Ranger PC was played poorly I would as the DM be incluned to simply freeze the character at the Ranger level extant at the time the qualifying ability was lost.

Cheers,
Gary
 

The six attributes used seemed to cover the spectrum of what a human consisted of in general manner.
They seem to have been on target, as there hasn't been a change in them for some years...