Traditionally, a fee tail was created by words of grant in the deed: "to N., and the heirs of his body." The crucial difference between thse words of conveyance and the words that created a fee simple, "to N., 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, such as that only the owner's children by a particular spouse could inherit it. 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 in order to create family settlements and 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 pur autre vie, 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 was productive of litigation.
Fee tail was a device tuned to the needs of family settlements in feudal times. In more mercantile eras, however, fee tail became an anachronism. Eighteenth century lawyers devised elaborate transactions involving collaborative lawsuits and legal fictions whose goal was to remove the conditions of fee tail from land and enable its free conveyance in fee simple. Fee tail has been abolished by statute in England and in most of the United States of America. In Louisiana the civil law doctrine of legitime already restricts owners from willing property out of their family when they die with children.