CCC 191

"These three parts are distinct although connected with one another. According to a comparison often used by the Fathers, we call them articles. Indeed, just as in our bodily members there are certain articulations which distinguish and separate them, so too in this profession of faith, the name "articles" has justly and rightly been given to the truths we must believe particularly and distinctly." In accordance with an ancient tradition, already attested to by St. Ambrose, it is also customary to reckon the articles of the Creed as twelve, thus symbolizing the fullness of the apostolic faith by the number of the apostles.