The Propædia is the first of three parts of the Encyclopædia Britannica, the other two being the 12-volume "Micropædia" and the 17-volume "Macropædia". The "Propædia" 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 encyclopaedia to replace the index, but when many people complained, Britannica put the index back in two books in 1985. The core of the "Propædia" is its Outline of Knowledge, which is supposed to give people a classification of what we know, but the "Propædia" also has some parts listing the staff members, advisors and contributors to all three parts of the "Britannica".