Proficiency Check: A Proficiency Check is accomplished in the same way as an
Ability Check. The player rolls 1d20, applies modifiers (if any) to
the result, and compares that number to the character’s score in
the Appropriate Ability for the proficiency being used. If the modified
die-roll result is less than or equal to the score of the Appropriate
Ability, the Proficiency Check is successful. (In certain
circumstances, the DM will make a Proficiency
Check die roll instead of the player, and he may or may not reveal
to the player the result of the attempt. See the description of the
direction sense proficiency for an example of this exception; the
DM may declare other exceptions of this sort when
he deems it appropriate.)

Any unmodified die roll of 19 or 20 on a Proficiency Check indicates
automatic failure, regardless of modifiers that would otherwise
bring the result down into the range needed for success.
Also, for the purpose of a Proficiency Check, any ability score
greater than 18 is treated as a score of 18. This means that a
character with an Appropriate Ability score of 18 or greater must
always make a successful Proficiency Check without the aid of
any beneficial modifiers, and that even a character with an Appropriate
Ability score of 18 or greater has at least a 10% chance
(2 in 20) of failing any Proficiency Check he attempts.
 

Proficiency Checks

In the tradition of the AD&D OA  rules, the
DSG presents rules for nonweapon proficiencies.
These proficiencies include areas of skill such as
weapon-making, carpentry, and mining, that can augment the
PC’s adventuring skills. Proficiencies are also presented for such
skills as climbing, fighting in the dark, and fire building. The latter
categories can actually help a character to survive in the midst of
an adventure.

In the cases of these adventuring proficiencies, players are
often called upon to make Proficiency Checks. Such checks are
made by rolling 1d20 and comparing the result to the character’s
rating in the given proficiency (rolls less than the rating are successful).
Although the exact nature of each proficiency is
described later, the following rules contain references to Proficiency
Checks. Consult the section on proficiencies to determine
the exact details in each case.

Proficiency Checks

Following in the footsteps of the OA book and
the DSG, this book describes special
skills, or proficiencies, not related to weapon use that a PC can acquire to give himself (and perhaps his comrades)
a better chance of success when performing a certain task
or attempting to cope with a hazard or a threat.
Sometimes a PC with proficiency can automatically
perform some task or function that is impossible for a nonproficient
character; at other times, he may be required to make a
Proficiency Check to determine his success or failure. The procedure
is essentially the same as for an Ability Check, since each
proficiency is directly related to a certain ability score. For more
details about Proficiency Checks, see the section on Wilderness
Proficiencies.

Success and Failure

Unlike a WP, the possession of a NWP does not always mean that the character can realize
the benefits of having a certain skill. On some occasions, depending
upon the particular proficiency or the circumstances surrounding
the use of the proficiency, it is necessary for a character
to make a successful Proficiency Check in order to be able to use
the skill.

A Proficiency Check is accomplished in the same way as an
Ability Check. The player rolls 1d20, applies modifiers (if any) to
the result, and compares that number to the character’s score in
the Appropriate Ability for the proficiency being used. If the modified
die-roll result is less than or equal to the score of the Appropriate
Ability, the Proficiency Check is successful. (In certain
circumstances, the Dungeon Master will make a Proficiency
Check die roll instead of the player, and he may or may not reveal
to the player the result of the attempt. See the description of the
direction sense proficiency for an example of this exception; the
Dungeon Master may declare other exceptions of this sort when
he deems it appropriate.)

Any unmodified die roll of 19 or 20 on a Proficiency Check indicates
automatic failure, regardless of modifiers that would otherwise
bring the result down into the range needed for success.
Also, for the purpose of a Proficiency Check, any ability score
greater than 18 is treated as a score of 18. This means that a
character with an Appropriate Ability score of 18 or greater must
always make a successful Proficiency Check without the aid of
any beneficial modifiers, and that even a character with an Appropriate
Ability score of 18 or greater has at least a 10% chance
(2 in 20) of failing any Proficiency Check he attempts.

<note: the OA prof. system is different from that in the DSG, and the WSG. (the latter two are similar to the 2e prof. system). to make matters, worse, there is a major error in the DSG>
<suggested fix: without changing any Official 1e rules, one can call Proficiencies "Skills", and convert all of the d20 numbers to a %.>
<for example, one would write Hunting 55% on their character sheet, in the Skill section>
<this is a lot more intuitive to new players of 1e: "Yes, Hunting 55% means that you have a 55% chance to Hunt successfully>
<check the above example>
<as well, resolve this, and be consistent: Hunting, or, Hunt? Swimming, or, Swim? Climbing, or, Climb? etc.>
<with due consideration, i prefer the verb above the noun, for skills: ie. Hunt, Swim, Climb, etc. -- the URLs are based on this>