On the section of 'ata' (in pinyin, 'zhi') in Shuowen Jiezi (Analytical Dictionary of Chinese Characters), there exists a description that '8 sun ('cun' in pinyin; 1 sun is about 3.03 centimeters) of shu ('zhou' in pinyin) was equalized with 1 ata, and that 10 sun of shu was equalized with 1 shaku ('chi' in pinyin; 1 shaku is about 30.3 centimeters).'
説文解字の「咫」に、「周尺の八寸を咫とし、十寸を尺とする」とある。