Original: 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.
Simplified: 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.