Rehnquist pointed out that Pennsylvania divides property interests into three categories. These categories are "the support estate, the surface estate, and the mineral estate." Each interest is deemed separate from the other and may have separate owners in fee simple.
According to Rehnquist, Pennsylvania classifies property interests into three types: the support estate, the surface estate, and the mineral estate.  Each of these categories is treated as a separate entity with its own owner, who can hold it in fee simple.