Commonplace books (or commonplaces) were a way to compile knowledge, usually by writing information into books. Such books were essentially scrapbooks filled with items of every kind: medical recipes, quotes, letters, poems, tables of weights and measures, proverbs, prayers, legal formulas. Commonplaces were used by readers, writers, students, and scholars as an aid for remembering useful concepts or facts they had learned. Each commonplace book was unique to its creator's particular interests. They became significant in Early Modern Europe.
Scholars have expanded this usage to include any manuscript that collects material along a common theme by an individual.
Commonplace books are not diaries nor travelogues, with which they can be contrasted: English Enlightenment philosopher John Locke wrote the 1706 book A New Method of Making a Common Place Book, "in which techniques for entering proverbs, quotations, ideas, speeches were formulated.
By the early eighteenth century they had become an information management device in which a note-taker stored quotations, observations and definitions. They were even used by influential scientists.
More information, including examples, on the Wikipedia page . There are a number of websites on the current creation and keeping of commonplace books.