An attack roll represents a character’s attempts to strike an opponent on the character’s turn in a round. When a character makes an attack roll, he or she rolls 1d20 and adds his or her attack bonus. If the result equals or beats the target’s Defense, the character hits and deals damage. Many modifiers can affect the attack roll.
A natural 1 (the d20 comes up 1) on the attack roll is always a miss. A natural 20 (the d20 comes up 20) is always a hit. A natural 20 is also always a threat—a possible critical hit.
If the character is not proficient in the weapon he or she is attacking with (the character doesn’t have the appropriate Weapon Proficiency feat), that character takes a –4 penalty on the attack roll.