"Upon [Robert Maynard ] Hutchins’s retirement in 1974, [Mortimer J.] Adler succeeded him as chairman of the Board of Editors. Under the stewardship of Adler, [William] Benton, and Charles E. Swanson (president of the company from 1967 to 1985), a vast editorial effort was assembled, resulting in the first publication of Britannica 3, or the 15th edition, in 1974. The new set consisted of 28 volumes in three parts serving different functions: the Micropædia: Ready Reference and Index, Macropædia: Knowledge in Depth, and Propædia: Outline of Knowledge. ..."In its entry on "Encyclopaedia", the section on Content arrangement concludes,
"The Propædia specifically was a reader’s version of the circle of learning on which the set had been based and was organized in such a way that a reader might reassemble in meaningful ways material that the accident of alphabetization had dispersed."While there is not a specific Britannica article on Propaedia, there is one at Wikipedia which includes the outline of its 10 Parts, 41 Divisions, and 167 Sections.