The ratio of the circumference of a circle to its diameter is about 3.14, and provided that we round off the second decimal place and regard the ratio as 3.2, the circumference of a circle whose diameter is 1 shaku becomes 3.2 shaku, and in ancient China, this 3.2 shaku was equalized with 4 ata.
円周率は約3.14であるが、これを3.2で近似すると、径1尺の円(数学)の円周は3.2尺となり、これを4咫とした。