Original: The 17-volume Macropædia is the third part of the Encyclopædia Britannica; the other two parts are the 12-volume Micropædia and the 1-volume Propædia.
Simplified: 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.