Dishu system

Dishu (Chinese: 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 was allowed one official wife, called a zhengshi (正室, pronounced seishitsu in Japanese, lit. "formal household") or Di wife (嫡妻), and her son was called the Di son (嫡子). In Tang Dynasty, any man who has more than one Di wives would be considered engaging illegal marriage, liable to one year of penal labor, and the woman involved would also receive a slightly less severe punishment unless she could prove being cheated into the marriage. Either case, the marriage would be annulled.

A secondary spouse was called a ceshi (側室, lit. "side household") or Shu wife (庶妻), and her son was called the Shu son (庶子). Di sons, regardless of their age, held much higher social status than the Shu sons, and the eldest Di son (嫡長子) held overriding priority over all other children of the house. An illegitimate son, born out of wedlock, was generally categorized as a Shu son, though he would have much lower status than those born to legitimate Shu wives.

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 or demote a zhengshi wife (以妻為妾) unless she had committed "seven misconducts for divorce" (七出).

1. Unfilial conducts (不順父母) — considered a sin as it is "immoral" (逆德)
2. Incapable of bearing sons (無子) — considered a sin as it "threatens bloodlines" (絕世)
3. Promiscuity (淫) — considered a sin as it "disrupts clan" (亂族)
4. Jealousy (妒) — considered a sin as it "disrupts family" (亂家)
5. Having severe illness (有惡疾) — considered a sin as it "hinders family rituals" (不可共粢盛)
6. Excessive gossiping (口多言) — considered a sin as it "instigates discord among relatives" (離親)
7. Theft (竊盜) — considered a sin as it is "against common good" (反義)

However, there were three conditions, known as "three exceptions" (三不去), that forbade a man from ever divorcing his wife even if she commits the above seven sins.

1. The wife has no parental family to return to after divorce (有所娶無所歸)
2. The wife has served three years of filial mourning for deceased parent(s)-in-law (與更三年喪)
3. The husband was poor upon marriage but now wealthy (前貧賤後富貴)

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. Any man who divorced his wife without legitimate reasons (the above-mentioned "seven misconducts") would be subjected to eighteen months of penal labor, and a further 100 strikes of caning if he violated the wife's protection under "three exceptions".

After the Song Dynasty, the difference between social status of Di and Shu wives/sons decreased.