Tō-on or tōon (唐音), are Japanese kanji readings imported from China by Zen monks and merchants during and after the Song dynasty, as a form of on'yomi (音読み). This period roughly corresponds with the mid-Heian to Edo periods of Japan. During the Muromachi period, they were referred to as sō-on (宋音). Together, they are collectively known as tōsō-on (唐宋音). Scholars divide tō-on into two groups: those brought by the Rinzai and Sōtō schools of Zen during the Kamakura period, sometimes referred to as sō-on, and those brought by the Ōbaku and Sōtō schools during the Edo period, sometimes more strictly delineated as tō-on. Tō-on readings are not systematic, as they were introduced piecemeal from China, often along with very specialized terminology. Examples of words and characters using tō-on readings include: chair (椅子), futon (蒲団), paper lantern (行灯), Ming (明) and Qing (清). The Ōbaku Zen school of Buddhism uses tō-on exclusively for liturgy.