Dishu system

Dishu (嫡庶) was an important legal and moral system involving marriage and inheritance in ancient East Asia.

Because upper-class men in ancient China, Korea and Japan often have more than one spouse to ensure birthing of an heir to their lands, properties and titles, a priority system was created to rank the offsprings' entitlement to inheritance. Under this system, a man's official wife was called a zhengshi (正室, pronounced seishitsu in Japanese, lit. "formal household") or Di wife (Chinese: 嫡妻), and her son was called the Di son (Chinese: 嫡子). A secondary spouse was called a ceshi (侧室, lit. "side household") or Shu wife (Chinese: 庶妻), and her son was called the Shu son (Chinese: 庶子). Di sons, regardless of their age, held much higher social status than the Shu sons, and the eldest Di son (嫡长子) held overriding priority over all other children of the house.

A bastard son, born out of wedlock, was generally categorized as a Shu son, though he would have much lower status than those born to legitimate marriages.

Tang dynasty law in China prescribed that if a Di son died, his eldest Di son (Di grandson) should be the successor, prioritized over all other members of the family; if a Di grandson could not be found, the Di son's next full-brother (born of the same zhengshi mother) should be the successor. If no Di offspring were available, a Shu son could be considered.

During most of the imperial China, a man could not divorce a zhengshi wife (休妻) until she had committed "seven crimes" (七出) — unfilial conducts (不顺父母), incapable of bearing sons (无子), promiscuity (淫), jealousy (妒), having severe illness (有恶疾), excessive gossiping to instigate discord (多言) and theft (窃盗). The Tang law prescribed that a man caught demoting his zhengshi wife to ceshi without good cause would be sentenced to two years of penal labor, and the zhengshi wife's status would be restored. After the Song Dynasty, the difference between social status of Di and Shu wives/sons reduced.