Diding 地丁, also called diding shui 地丁稅 or diding yin 地丁銀, was the unfied field-and-poll tax. After a long preparatory phase, in which both the field tax (tianfu 田賦, difu 地賦) and the poll tax (dingfu 丁賦, dingyin 丁銀) were monetized, the Kangxi Emperor 康熙 (r. 1654-1722) decided to merge both. This transformation was called tanding rumu 攤丁入畝, tanding rudi 攤丁入地, or diding heyi 地丁合一.
In the late 14th century peasants had still to pay their field tax in kind, and to deliver labour for official projects (see yaoyi 徭役). With the single-whip method (yitiaobian fa 條鞭法后), these two types of tax were merged, and peasants were allowed to pay a certain amount of money, instead of delivering grain or working for the government. This monetized poll tax was called dingyin 丁銀, but it was still separately registered from the field tax.
The early Qing government took over this procedure and levied (kezheng 課征) these taxes separately. From 1724 on the two types of liabilites were collected in one instance. The merged tax was called diding or diding qianliang 地丁錢糧 (qianliang 錢糧 actually means "cash and grain", but can also just mean "money"). The diding was levied at standard quotas, yet because the field tax had differed from region to region, there was no uniform method of payment: in some places part of the tax was still delivered in grain. The field tax depended on the size and quality of the field, and per 1 tael (liang 兩) of silver for the field tax, a surplus of 0.2 liang (or 2 qian 錢, see money) was to be paid for the poll tax. The general surplus of 20 per cent on the field tax allowed to save bureaucratic paperwork to determine the number of male—and thus tax-liable— heads in each household.
The fusing of the field and the poll tax was a further step of rationalizing earlier simplifications of the tax system, like the twice-taxation system (liangshui fa 兩稅法) of the Tang period 唐 (618-907) and the single-whip method of the Ming dynasty 明 (1368-1644).