The PropÃ dia is supposed to be a book that organizes the things in the Britannica , and a guideline to the index of the other two parts . It was introduced in 1974 with the 15th edition of the encyclopedia to replace the index , but when many people complained , Britannica put the index back in two books in 1985 . Introduced in 1974 with the 15th edition , the PropÃ dia and MicropÃ dia were intended to replace the Index of the 14th edition ; however , after widespread criticism , the Britannica restored the Index as a two-volume set in 1985 .