1: Dovedale

When a character uses a proficiency, either the attempt is automatically successful, or the character must roll a proficiency check. If the task is simple or the proficiency has only limited game use (such as cobbling or carpentry), a proficiency check is generally not required. If the task the character is trying to perform is difficult or subject to failure, a proficiency check is required. Read the descriptions of the proficiencies for details about how and when each can be used.
If a proficiency check is required, Table 37 lists which ability is used with each proficiency. Add the modifier (either positive or negative) listed in Table 37 to the appropriate ability score. Then the player rolls 1d20. If the roll is equal to or less than the character's adjusted ability score, the character accomplished what he was trying to do. If the roll is greater than the character's ability score, the character fails at the task. (A roll of 20 always fails.) The DM determines what effects, if any, accompany failure.
Of course, to use a proficiency, the character must have any tools and materials needed to do the job. A carpenter can do very little without his tools, and a smith is virtually helpless without a good forge. The character must also have enough time to do the job. Certainly, carpentry proficiency enables your character to build a house, but not in a single day! Some proficiency descriptions state how much time is required for certain jobs. Most, however, are left to the DM's judgment.