What does the Torah say about the firstborn's inheritance rights?

Deuteronomy 21:15-17 sets out the Torah's core rule for a firstborn son's inheritance: "he shall acknowledge the firstborn, the son of the unloved, by giving him a double portion of all that he has, for he is the firstfruits of his strength. The right of the firstborn is his." The passage's specific scenario is a man with two wives, one loved and one unloved, whose firstborn son happens to be the child of the unloved wife; the law forbids the father from favoring a younger son of the preferred wife over the true firstborn simply because of personal affection, making inheritance rights a matter of birth order rather than parental sentiment. In practice, "double portion" means the firstborn receives twice the share of any other single heir - with two sons the firstborn gets two-thirds of the estate and the second son one-third, with three sons the firstborn gets half and the other two split the remainder equally, and so on as the number of heirs grows. This inheritance law is distinct from, though related to, the Torah's broader concept of the bekhor (firstborn), which also carries cultic significance: Exodus 13:2 commands that Israel "consecrate to me all the firstborn... of man and of beast," and firstborn sons were originally understood to hold a priestly role before that role was formally transferred to the tribe of Levi (Numbers 3:12-13) in exchange for the Levites' dedicated service at the tabernacle. Firstborn animals were also subject to specific rules - non-kosher animals like donkeys had to be redeemed with a lamb or have their neck broken (Exodus 13:13), while firstborn kosher livestock were generally offered as sacrifices. Together these laws reflect a consistent Torah principle: the status of "firstborn" carries real, legally enforceable privilege and obligation, but that status is fixed by the order of birth rather than being something a parent can reassign by preference.