Two: Definition
Commonplaces 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 aide-mémoire for remembering useful concepts or facts.
#HeyPresstoConf20

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.
“Commonplace” is a translation of the Latin locus communis (from Greek tópos koinós, see literary topos) which means “a general or common topic”, such as a statement of proverbial wisdom.