§ 6-2.1 Estates in severalty, joint tenancy, tenancy by the entirety and in common Estates  as  to  the  number of persons owning an interest therein are  classified as follows: (1)  In severalty. (2)  Joint tenancy. (3)  Tenancy in common. (4) Only as to real property and, on and after January first, nineteen  hundred ninety-six, as to the shares of stock of a cooperative apartment  corporation  allocated  to  an  apartment  or  unit  together  with  the  appurtenant proprietary lease, tenancy by the entirety.