This section is from the book "The Law Of Real Property and Other Interests In Land", by Herbert Thorn Dike Tiffany. Also available from Amazon: A Treatise on the Modern Law of Real Property and Other Interests in Land .
The expression "attornment" has been used in two senses. In one sense it refers to the recognition by the tenant of the transferee of the reversion as his landlord, the necessity of which recognition has been generally dispensed with by statute.25 In the other sense, which is the one of interest in the present connection, the expression serves to describe the acknowledgment, by one previously in possession of land, that he is tenant to another, not a transferee of a reversion. That is, if one in possession of land, whether he is rightfully or wrongfully in possession, agrees to hold the land as tenant of another, he is said to attorn to such other. Such an attornment to another is, in legal effect, equivalent to the acceptance of a lease from such other. As one who obtains possession of land by permission of another for a limited period only becomes such other's tenant, so one already in possession who retains possession by permission of such other becomes his tenant. In either case there is a lease or demise of the land if the one person thereafter holds the land "under" the other, that is, by force of his acknowledgment that he is tenant of the other, and it is entirely immaterial that words of leasing are not used, the demand for or acceptance of the acknowledgment being in effect a giving of permission to occupy, sufficient at least to create a tenancy at will. Even in the case of a formal lease to a person already in possession in his own right or under a person other than the lessor, the lessee may properly be regarded, by his acceptance of the lease, as making an attornment to the lessor.26