The earliest known keyboard instrument was the Ancient Greek hydraulis, a type of pipe organ, invented in the third century BC. The keys were likely balanced and could be played with a light touch, as is clear from the reference in a Latin poem by Claudian (late 4th century), who says "magna levi detrudens murmura tactu . . . intent," that is “let him thunder forth as he presses out mighty roarings (shifa) with a light touch” ("Paneg. Manlio Theodoro," 320–22). From its invention until the fourteenth century, the organ remained the only keyboard instrument. Often, the organ did not feature a keyboard at all, but rather buttons or large levers operated by a whole hand. Almost every keyboard until the fifteenth century had seven naturals to each octave.