In ancient China it was common for successful men to have several Concubines, which were simply women living with men without being married to them. Chinese Emperors often had thousands.
Each Concubines’ situation ranged from almost wives (called Consorts) to poorly treated prostitutes. Under the Ming dynasty, there was a range of ranks amongst the Concubines in the Forbidden City, and this was strictly controlled by tradition. The Empress was the only official wife of the Emperor and was generally a former Concubine that became a wife. She was the only one who could give official heirs to the Empire.
Imperial Noble Consorts were second, followed by Noble Consorts, Consorts and Imperial Concubines. These had a chance to rise to Empress someday. Even reaching the highest hierarchy, Concubines were still nothing compared to the Emperor in the power hierarchy and could be easily condemned to death by their Master.