This section is from the book "Real Property, An Introductory Explanation Of The Law Relating To Land", by Alfred F Topham. Also available from Amazon: The New Law Of Property.
So far we have dealt with a general entail only; that is a grant to "A and the heirs of his body" - without restriction as to the sex of the heirs, and without naming their mother.
But land may be granted in special tail, "to A and the heirs male of his body" which is called an estate in tail male, or "to A and the heirs female of his body" which is called an estate in tail female, or to "A and the heirs of his body by his wife B." Then the land can only descend to A's children by his wife B, and if B should die without issue, it will become impossible for A to have issue who can inherit under the entail.
A is then called a tenant in tail after possibility of issue extinct.
If the land is granted to A and the heirs of his body generally it is impossible for hirn to become a tenant in tail after possibility of issue extinct: for the law will never consider that a man is too old to have issue, and therefore if A is 90 years old and unmarried, the law will always presume that he might marry and have children.
A tenant in tail after possibility (as he is more shortly called) cannot bar the entail. He is in much the same position as a life tenant, but he is not reponsible for waste (as to which see Chapter VI (Tenant For Life. Section I. How A Life Estate Is Created), s. 2.
Similarly estates tail granted as a reward for services to the State, cannot be barred.
E.g. The Blenheim estates granted to the Duke of Marlborough.