Fee tail or entail is an obsolete term of art in common law. It describes an estate of inheritance in real property which cannot be sold, devised by will, or otherwise alienated by the owner, but which passes by operation of law to the owner's heirs upon his death. The term fee tail is derived from the Middle Latin feodum talliatum, which means "cut-short fee." The Statute of Westminster II, passed in 1285, created and stereotyped this form of estate. The new law was also formally called the statute De Donis Conditionalibus (Concerning Conditional Gifts).
Traditionally, a fee tail was created by words of grant in the deed: "to A and the heirs of his body." The crucial difference between the words of conveyance and the words that created a fee simple, "to A and his heirs," is that the heirs "in tail" must be the children begotten by the landowner. It was also possible to have "fee tail male," which only sons could inherit, and "fee tail female," which only daughters could inherit; and "fee tail special," which had a further condition of inheritance, usually restricting succession to certain "heirs of the body" and excluding others. Land subject to these conditions was said to be entailed or in tail. The restrictions themselves were entailments.
Fee tail was formerly used during feudal times by landed nobility in order to create family settlements and to make certain that the land stayed in the family. From the foregoing, attempting to mortgage land in fee tail would be risky and uncertain, since at the death of the owner the land passed by operation of law to children who had no obligation to the mortgage lender and whose interest was prior in right over the mortgage. Similarly, the largest estate an owner in fee tail could convey to someone else was a life estate, since the grantee's interest again terminates automatically when the original owner died. If all went as planned, it was impossible for the family to lose the land, which was the idea.
Things do not always go as planned, however. Owners of land in tail occasionally had "failure of issue" --- that is, they had no children surviving them at the time of their own deaths. In this situation, theoretically the entailed land went back up and through the family tree to descendants of former owners who were entitled to inherit, or to the last owner in fee simple. This situation produced complicated litigation.
Fee tail was a device tuned to the needs of family settlements in the thirteenth century, but it was never popular with the monarchy, the merchants, or many entailed holders themselves who wished to sell their land. In more mercantile eras, fee tail became rare. As early as the fifteenth century, lawyers devised an elaborate action called "Common Recovery," which used collaborative lawsuits and legal fictions to remove the conditions of fee tail from land and enable its free conveyance in fee simple.
Fee tail was abolished by statute in England (as a legal estate) in 1925 and has been in all but four states in the United States: Delaware, Massachusetts, Maine, and Rhode Island. New York, for example, abolished it in 1782. In most states it is provided that an attempt to create a fee tail results in a fee simple. Even in those four states that still allow fee tail, the estate holder may convert his fee tail to a fee simple during his lifetime by executing a deed. Many other states never recognized the fee tail estate at all as most of the land in the United States of America was deemed allodial. In Louisiana, the sole civil law jurisdiction in the US, the doctrine of legitime restricts owners from willing property out of their family when they die with children.
An entail can still exist in England and Wales as an equitable interest, behind a strict settlement, but the legal estate is vested in the current 'tenant for life' or other person immediately entitled to the income, but on the basis that any capital money arising must be paid to the settlement trustees. A tenant in tail in possession can bar his entail by a simple disentailing deed, which does not now have to be enrolled. A tenant in tail in reversion (i.e. a future interest where the property is subject to prior life interest) needs the consent of the life tenant and any 'special protectors' to vest a reversionary fee simple in himself. Otherwise he can only create a base fee; a base fee only confers a right to the property on its owner, when its creator would have become entitled to it; if its creator dies before he would have received it, the owner of the base fee gets nothing.

See also

Majorat
Ordynat
Rule in Wild's Case