褚遂良
Today, among the existing copies of the preface, a few outstanding imitations were attributed to Feng Chengsu (617 – 672), Ouyang Xun (557 – 641), Yu Shinan (558 – 638) and Chu Suiliang (596 – 659) from the Tang dynasty. Feng’s version, also called the "Shenlong" version (Figure 2), was regarded as the closest resemblance to the original. Feng, as the royal copyist, mastered the calligraphy tracing technique which was named Xiang Tuo. The method requires the copyist to stay in a dark room and stick the artwork against a window where the sunlight can shine through the paper to expose every detail of the characters; Another empty paper is then attached to the original piece to trace the outline of each stroke meticulously before filling in with ink. The "Shenlong" version is currently preserved at the Palace Museum in Beijing.