This section is from the book "Practical Real Estate Methods For Broker, Operator & Owner", by Thirty Experts. Also available from Amazon: Practical Real Estate Methods for Broker, Operator, Owner.
When a tenant or lessee disposes of his entire interest under a lease, it is called an assignment. When a tenant leases the whole or any part of the premises for a shorter period than the term of his own lease, it is called a sub-lease. Although a tenant, by the lease under which he holds, might be prohibited from making an assignment, he would, nevertheless, have the right to make a sub-lease. The distinction between an assignment and a sub-lease is that in an assignment we find a transfer of the tenant's whole interest, and that in a sub-lease the tenant has reserved for himself a reversion. When a lessee assigns his entire interest, his assignee, as long as he is in possession of the property, is liable under the lease. He is said to be in privity of estate with the landlord. The original lessee also remains liable because of the agreements contained on his part in the original lease. The original lessee is said to be liable because of privity of contract. Thus, in cases of assignment two persons are liable to the landlord - the original lessee by privity of contract, and the assignee by privity of estate.