Macropædia

The 17-book Macropædia is the third part of the "Encyclopædia Britannica", and the other two parts are the 12-book "Micropædia" and the 1-book "Propædia". The name comes from the word "macro" and "encyclopaedia". Macro means big, and so, the Macropædia is a big encyclopaedia. The Macropaedia has articles that are very detailed, but one article is usually about a very large topic. It is the opposite of the Micropædia, which has articles that are short. 
The "Macropædia" was introduced in the 15th edition of the Encyclopædia Britannica in 1974 with 19 book having 4,207 articles. In 1985, these articles were combined and made smaller into 17 books with about 700 articles, which can be as short as 2 pages, and as long as 310 pages. The longest article, on the United States, was from the combining of the 50 articles on each state. 
Since it was created, it has not stayed the same. New articles are always being added, older articles can be split, shortened, merged, or even deleted.