The Renaissance tradition of recondite emblem books had, in the hands of the 17th-century Dutch – almost universally literate in the vernacular, but mostly without education in the classics – turned into the popularist and highly moralistic works of Jacob Cats, Roemer Visscher, and others, often based in popular proverbs. The illustrations to these are often quoted directly in paintings, and since the start of the 20th century art historians have attached proverbs, sayings and mottoes to a great number of genre works. Another popular source of meaning is visual puns using the great number of Dutch slang terms in the sexual area: the vagina could be represented by a lute ("luit") or stocking ("kous"), and sex by a bird ("vogelen"), among many other options, and purely visual symbols such as shoes, spouts, and jugs and flagons on their side.