The transformation from clay stamp to affixed seal was completed during the Sui dynasty. The Tang dynasty followed the seal system from the Sui dynasty and started forming a new system of official seals. The calligraphic inscription of Kaishu used for both official and private seals appeared during the Tang and Song dynasties. Signature became popular during the Song dynasty, which enriched expressive patterns of seals. During the periods of the Liao, Xixia, Jin and Yuan dynasties,various ethnic writings were used for seal-carving but they still imitated the style of Chinese seals.