Heir and spare
Heir and spare, or the heir and the spare, is a term referring to first-born and second-born children, preferably male, in patrilineal inheritance systems. The first-born is heir apparent or heir presumptive. The second-born is redundancy in case there should be a catastrophic failure of Plan A. The brutal clarity of this winner-takes-all system contrasts with other, more ambiguous systems where heirs are never told what, how much, or if they will inherit at all. Historically, it was the duty of women to deliver these heirs for the sake of the  perpetuation of the extant economic and social system. For example, before Geneviève d'Arconville began her scientific work, she married and, while still a teenager, "had produced for him an heir, a spare, and even a third son for good measure." In current affairs, the phrase was the inspiration for the title of Prince Harry, Duke of Sussex's 2022 memoir, Spare. Conceiving children, sometimes called savior siblings, to provide bone marrow or organs for their sickly older siblings, has been derisively described by a derivative phrase: "the heir and the spare parts." See also
 Male heir
 Birth order

References

Further reading 
 Morgan, E. M. (2009). The heir and the spare: Impact of birth order on risk attitudes, discount rates, and behaviors (Order No. 3366549). Heirs to the throne
Patriarchy
Inheritance
Legal history
Real property law
Succession
Hereditary monarchy
Order of succession
Kinship and descent
Sibling
English-language idioms