Iconography (from Greek 'eikon': image, 'graphein': to write) is the branch of art history concerned with identifying and interpreting the subjects and symbols in visual art. Every period and culture has its own visual language: in medieval and Renaissance Christian art, a lamb represents Christ, a lily represents purity, a skull represents mortality, a dog represents fidelity. To understand a painting as its original audience did requires reading this vocabulary. Erwin Panofsky codified the discipline in his Studies in Iconology (1939), distinguishing between the pre-iconographic level (recognising what objects are), the iconographic level (identifying their symbolic meaning), and the iconological level (understanding what they mean culturally and historically). Iconographic misreadings are common — Turner's paintings were often described as failures of technique by critics who did not understand what he was attempting.