In experiments where DNA is pulled at constant force and twisted, a mixed state of extended twisted DNA and plectonemically supercoiled DNA is easily obtained. I will review the thermodynamics of this "state coexistence" phenomenon. A particularly elegant use of plectoneme-extended DNA coexistence is as a source of constant torque for single-DNA experiments. I will discuss the estimation of torques and free energies in such experiments, and I will provide some useful closed-form formulae, most notably for the torque in DNA as a function of force. I will review experiments where estimation of DNA torques allowed study of rotational friction during DNA relaxation by type IB and IC topoisomerases. I will also discuss experiments where DNA torques were directly measured rather than estimated. Reference: J.F. Marko, Torque and dynamics of linking number relaxation in stretched supercoiled DNA, Phys. Rev. E 76, Art. 021926 (2007)