Concubine implies a lower status in a household hierarchy compared to a wife. The concubine could not dine at the same table as the man's wife.
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Not Official
A concubine's relationship with a man isn't officially recognized like a marriage. Even though she lived with him, she remained a concubine, not a spouse.
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Cultural Specificity
The term is often tied to certain cultures or historical periods—be aware of context. In that culture, concubines were common, but the practice is now outdated.