They have been kept from antiquity, and were kept particularly during the Renaissance and in the nineteenth century. Such books are essentially scrapbooks filled with items of every kind: recipes, quotes, letters, poems, tables of weights and measures, proverbs, prayers, legal formulas. Commonplaces are used by readers, writers, students, and scholars as an aid for remembering useful concepts or facts. Each one is unique to its creator’s particular interests but they almost always include passages found in other texts, sometimes accompanied by the compiler’s responses.
Source: Commonplace book – Wikipedia