The value of tax shields depends only on the net increases of debt

The value of tax shields depends only on the nature of the stochastic process of the net increases of debt. The value of tax shields in a world with no leverage cost is the tax rate times the current debt plus the present value of the net increases of debt. By applying this formula to specific situations, we show that Modigliani-Miller (1963) should be used when the company has a preset amount of debt, Fernández (2004) when the company maintains a fixed book-value leverage ratio, and Miles-Ezzell (1980) when the company maintains a fixed market-value leverage ratio.