In this work, which is a natural continuation of [1] and [2], we show, that the coexistence of the maximal number of real spectral values of generic single-delay retarded second-order differential equations guarantees the realness of the rightmost spectral value. This is interpreted from a control theory point of view as, using a delayed PD controller, one can stabilize a second-order delay differential equation by assigning the maximal number of negative roots of the corresponding characteristic function (which is a quasipolynomial). We give a necessary and sufficient condition for the rightmost root to be negative and thus guarantee the exponential decay rate of the system solutions. We illustrate the proposed design methodology in the delayed PD control of the harmonic oscillator.