The original gauge for songs’ popularity was the figure for sales of a given number’s sheet music. To catch the eye of an amateur or professional musician shopping for new material to play at home or onstage, publishers relied on illustrations. Library of Congress; Lebrecht Music Arts/Bridgeman Images; Sheridan Libraries/Levy/Gado/Getty Images(3); Library of Congress; Sheridan Libraries/Levy/Gado/Getty Images; New York Public Library; National Archives)