Cooperative ownership of an apartment unit means that the apartment owner has purchased shares in a corporation which holds title to the entire apartment building. The cooperative owner is, in essence, a shareholder in a corporation whose principal asset is a building. In return for his stock in the corporation, the owner receives a proprietary lease entitling him to occupy a specific unit in the building. He thus occupies but does not own his unit. He must pay his prorated share of the corporation’s expenses, which include mortgage charges, real estate taxes, maintenance, payroll and the like. The owner can deduct for tax purposes his share of the taxes and interest charged.