Proficiency. Proficiency is an important concept. Characters have a proficiency bonus determined by class level. A character's class and background determines a character's various proficiencies.
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If you are proficient with a weapon used in an attack, then you add your proficiency bonus to the attack roll (but not to damage).
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If you use armor or a shield that you lack proficiency with, you have disadvantage on any ability check, saving throw, or attack roll that involves Strength or Dexterity, and you can’t cast spells.
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If you are proficient in a skill or tool that applies to an ability check, then you add your proficiency bonus to the check result.
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If you are proficient in a type of saving throw, then you add your proficiency bonus to your saving throw result.
Your proficiency bonus can’t be added more than once. For example, if two different rules say you can add your proficiency bonus to a Wisdom saving throw, you nevertheless add the bonus only once.
Occasionally, your proficiency bonus might be multiplied or divided (doubled or halved, for example) before you apply it. See, for example, the rogue’s Expertise feature.