Simplify the following text for a Grade 9 native English-speaking person.
Original: The current 15th edition has a unique three-part structure : a 12-volume MicropÃ dia of short articles ( generally fewer than 750 words ) , a 17-volume MacropÃ dia of long articles ( two to 310 pages ) and a single PropÃ dia volume to give a hierarchical outline of knowledge .
Simplified: Since 1985 , the Britannica has had four parts : the MicropÃ dia , the MacropÃ dia , the PropÃ dia , and a two-book index . The Britannicas articles are found in the MicropÃ dia and MacropÃ dia , which contain 12 and 17 books , respectively , each book having about one thousand pages .