The Biden proposal applies only to very large corporations, but all corporations must comply with the corporate tax. The minimum tax rate of 15% would apply to book income, but the regular corporate income tax is based on taxable income. Assume that a company that is subject to the minimum corporate tax has a regular corporate tax liability that is less than 15% of its book income. The company's tax liability will be increased by the difference between the lower corporate tax amount and the amount equal to 15% of book income, so that the tax owed is equal to 15% of its book income. If a company's regular corporate income tax liability exceeds 15% of its book income, the company will owe the regular corporate income tax amount.