<p>In this paper, the reduction formulas for the mutual coherence matrix and the cross-spectral density matrix are derived when all polarization combinations exhibit cross-spectral purity in a wavefront-folding interferometer for nonstationary electromagnetic light. By utilizing it, the reduction formula also applies to the elements of the mutual coherence matrix or the cross-spectral density matrix normalized by the respective components of intensity or spectral density, respectively. This confirms the equality of spatially dependent elements of the correlation matrices in the space-time and space-frequency domains. In contrast to the scalar fields, where the complex and spectral degrees of coherence follows the reduction property, the squared space-time and space-frequency electromagnetic degrees of coherence, in general do not exhibit the reduction property. But for a fully coherent beam, the squared electromagnetic degree of coherence follows the reduction formula. Finally, the propagation properties of cross-spectrally pure electromagnetic fields and a method for their generation are also discussed.</p>