In English common law, fee tail or entail is a form of trust, created by deed or settlement by a 'grantor', that restricts the sale or inheritance of an estate in real property and prevents that property from being sold, wasted, devised by will, or otherwise alienated by a 'tenant-in-possession', and instead causes it to pass automatically and entirely, by operation of law, to an heir, or 'tenant in tail', as determined by the settlement deed. The terms fee tail and tailzie are from Medieval Latin feodum talliatum, which means "cut(-short) fee". In its original form, the 'tenant in possession' is also a 'tenant for life' and has exclusive control over all fruits of the property while they live, but no prospect of coming into ownership of the property in 'fee simple'; the 'tenant in tail' can look forward to enjoying the property in 'fee simple' when the tenant in possession dies, but in the meanwhile has no access to the fruits of the property, except as specified in the 'settlement'.

Fee tail deeds are in contrast to "fee simple" deeds, possessors of which have an unrestricted title to the property, and are empowered to bequeath or dispose of it as they wish (although it may be subject to the allodial title of a monarch or of a governing body with the power of eminent domain). Equivalent legal concepts exist or formerly existed in many other European countries and elsewhere; in Scots law tailzie was codified in the Entail Act 1685.

Most common law jurisdictions have abolished fee tails or greatly restricted their use. They survive in limited form in England and Wales, but have been abolished in Scotland, Ireland, and all but four states of the United States.

The fee tail was devised to enable a patriarch to perpetuate his blood-line, family-name, honour and armorials in the persons of a series of capable and independent male descendants. By keeping his estate intact in the hands of one heir alone, in an ideally indefinite and pre-ordained chain of succession, his own wealth, power and family honour would not be dissipated amongst several male lines, as became the case for example in Napoleonic France by operation of the Napoleonic Code which gave each child the legal right to inherit an equal share of the patrimony, where a formerly great landowning family could be reduced in a few generations to a series of small-holders or peasant farmers. It approaches the true corporation, who is a legal person who does not die but continues to exist independently of the directors and owners.

Indeed, as a trust, whilst trustees may die, replacements are appointed. The trust itself continues, in perpetuity or not. As many jurisdictions do not allow perpetual entities in private interests, 'fees tail' are commonly restricted by a rule against perpetuities to extend only for a limited number of future generations.

An entail also had the effect of disallowing illegitimate children from inheriting. It created complications for many propertied families, especially from about the late 17th to the early 19th century, leaving many individuals wealthy in land but heavily in debt, often due to annuities chargeable on the estate payable to the patriarch's widow and younger children, where the patriarch was swayed by sentiment not to establish a strict concentration of all his wealth in his heir leaving his other beloved relatives destitute. Frequently in such cases the generosity of the settlor left the entailed estate as an uneconomical enterprise, especially during times when the estate's fluctuating agricultural income had to provide for fixed sum annuities. Such impoverished tenants-in-possession were unable to realise in cash any part of their land or even to offer the property as security for a loan, to pay such annuities, unless sanctioned by private Act of Parliament allowing such sale, which expensive and time-consuming mechanism was frequently resorted to. The beneficial owner (or tenant-in-possession) of the property in fact had only a life interest in it, albeit an absolute right to the income it generated, the legal owners being the trustees of the settlement, with the remainder passing intact to the next successor or heir in law; any purported bequest of the land by the tenant-in-possession was ineffective.

Fee tail was established during feudal times by landed gentry to attempt to ensure that the high social standing of the family, as represented by a single patriarch, continued indefinitely. The concentration of the family's wealth into the hands of a single representative was essential to support this process. Unless the heir had himself inherited the personal and intellectual strengths of the original great patriarch, often a great warrior, which alone had brought him from obscurity to greatness, he would soon sink again into obscurity, and required wealth to maintain his social standing. This feature of English gentry and aristocracy differs from the aristocracy which existed in pre-Revolution France, where all sons of a nobleman inherited his title and were thus inescapably members of a separate noble caste in society. Little-known, France then had one of the lowest ratios of noble families to population, in Europe. The accepted rule was however largely compensated by written or notarized wills which allowed fathers to favour, within certain limits, a first-born son. In England, primogeniture provided that an estate would be inherited entirely by the first-born legitimate son of a nobleman and that, accordingly, subsequent sons were born as mere gentlemen and commoners. Without the support of wealth, these younger sons might quickly descend into obscurity, and often did. On this eldest son was concentrated the honour of the family, and to him alone was granted all its wealth to support his role in that regard, by the process of the fee tail.

The effects of English primogeniture and entail have been significant plot details or themes in a number of notable works of English literature. (See some examples cited below.)