Under the Taiho Code promulgated in the Nara period, the weight of 1 ryo was set around 41to 42 grams, following the standard of the Sui Dynasty (or the early Tang Dynasty) China, and when the weight of 1 ryo was reduced by around 11 percents into 37.3 grams in Tang Dynasty, Japan followed China again and took almost the same standard.
奈良時代に公布された大宝律令では隋代（唐代初期か）の一両に準じて、おおむね41～42グラムくらいであったが、唐代になり11%程度減少し37.3グラムとなり、日本国内でもこれに近い値となった。