The rules of inheritance differ between societies and have changed over time.
In common law jurisdictions, an heir is a person who is entitled to receive a share of the decedent's property via the rules of inheritance in the jurisdiction where the decedent died or owned property at the time of death. Strictly speaking, one becomes an heir only upon the death of the decedent. It is improper to speak of the "heir" of a living person, since the exact identity of the persons entitled to inherit are not determined until the time of death. In a case where an individual has such a position that only her/his own death before that of the decedent would prevent the individual from becoming an heir, the individual is called an heir apparent. There is a further concept of jointly inheriting, pending renunciation by all but one, which is called coparceny.
In modern legal use, the terms inheritance and heir refer only to succession of property from a decedent who has died intestate (that is, without a will). It is a common mistake to refer to the recipients of property through a will as heirs when they are properly called beneficiaries, devisees, or legatees.
Different cultures have had different rules of inheritance.
Both anthropology and sociology have made detailed studies in this area. Many cultures feature patrilineal succession, also known as gavelkind, where only male children can inherit. Some cultures also employ matrilineal succession only passing property along the female line. Other practices include primogeniture, under which all property goes to the eldest child, or often the eldest son, or ultimogeniture, in which everything is left to the youngest child. Some ancient societies and most modern states employ partible inheritance, under which every child inherits (usually equally). Historically, there were also mixed systems:
Employing differing forms of succession can affect many areas of society. Gender roles are profoundly affected by inheritance laws and traditions. Primogeniture has the effect of keeping large estates united and thus perpetuating an elite. With partible inheritance large estates are slowly divided among many descendants and great wealth is thus diluted, leaving higher opportunities to individuals to make a success. (If great wealth is not diluted, the positions in society tend to be much more fixed and opportunities to make an individual success are lower.)
Inheritance can be organized in a way that its use is restricted by the desires of someone (usually of the decedent). An inheritance may have been organized as a fideicommissum, which usually cannot be sold or diminished, only its profits are disposable. A fideicommissum's succession can also be ordered in a way that determines it long (or eternally) also with regard to persons born long after the original descendant. Royal succession has typically been more or less a fideicommissum, the realm not (easily) to be sold and the rules of succession not to be (easily) altered by a holder (a monarch).
In more archaic days, the possession of inherited land has been much more like a family trust than a property of an individual. Even in recent years, the sale of the whole of or a significant portion of a farm in many European countries required consent from certain heirs, and/or heirs had the intervening right to obtain the land in question with same sales conditions as in the sales agreement in question.