Definition of 'After-Tax Basis'
A comparison of the net yields produced by taxable and tax-exempt bonds. After-tax basis is most often used when comparing between bonds used to finance private business endeavors (corporate bonds) and bonds used to finance public projects (tax-free municipal bonds). Because one must pay taxes on income derived from corporate bonds, the yield on those bonds is necessarily less than the stated interest rate, whereas tax-exempt municipal bonds can be evaluated at face value.
Investopedia explains 'After-Tax Basis'
While the after-tax basis evaluation of taxable and tax-exempt bonds would seem to be very straightforward, it's important to remember that the amount of tax one pays can be a function of one's income, as well as the performance of his/her other holdings.
For example, an investor in a 35% tax bracket receives what amounts to 6.5% interest on a corporate bond with a 10% yield, whereas an investor in a 15% tax bracket, receives 8.5%. Similarly, an investor with market losses to totally or partially offset his/her market gains, will likewise realize more of the listed yield on a corporate bond than an investor with no such losses.