Claim: Keturah was a the mistress of a Biblical patriarch.

Concubinage is an interpersonal relationship in which a person engages in an ongoing sexual relationship with another person to whom he/she is not or can not be married to in the full extent of the local meaning of marriage . The inability to marry may be due to multiple factors such as differences in social rank status , an existing marriage , religious prohibitions , professional ones -LRB- for example Roman soldiers -RRB- or a lack of recognition by appropriate authorities . The woman in such a relationship is referred to as a concubine .   The prevalence of concubinage and the status of rights and expectations of a concubine have varied between cultures as well as the rights of children of a concubine . Whatever the status and rights of the concubine , they were always inferior to those of the wife , and typically neither she nor her children had rights of inheritance . Historically , concubinage was frequently entered into voluntarily -LRB- by the woman or her family -RRB- as it provided a measure of economic security for the woman involved . Involuntary or servile concubinage sometimes involved sexual slavery of one member of the relationship , usually the woman . Nevertheless , sexual relations outside marriage were not uncommon , especially among royalty and nobility , and the woman in such relationships was commonly described as a mistress . However , the children of such relationships were counted as illegitimate and were barred from inheriting the father 's title or estates , even when there was an absence of any legitimate heirs .   While various forms of long-term sexual relationships and co-habitation short of marriage have become increasingly common in the Western world , these are generally not described as concubinage . The terms concubinage and concubine are used today primarily when referring to non-marital partnerships of earlier eras . In modern usage , a non-marital domestic relationship is commonly referred to as co-habitation -LRB- or similar terms -RRB- , and the woman in such a relationship is generally referred to as a girlfriend , lover or -LRB- life -RRB- partner .