Chinese calligraphy originated around 1800 BC with the earliest forms being pictographs found on oracle bones. Calligraphy evolved through the bronze script era where inscriptions were made on cast bronze objects from 1100 BC. Paper was invented in AD 105 by Cai Lun, allowing writing to be done on a portable surface rather than bamboo strips. Chinese calligraphy is distinguished by its emphasis on the structure, stroke, and precise control of the brush which has been refined over 2000 years of practice in regular script styles. Seals known as "chops" were also developed for leaving personalized stamped impressions.