We describe the modes of a Fabry–Perot cavity made of two-dimensional photonic crystal guided-mode resonance mirrors (${\mathrm{Si}}_3{\mathrm{N}}_4/\text{air}$), and compare it with an ideal Fabry–Perot cavity and a cavity made of Bragg mirrors. As the evolution of modes is analyzed, a lower tuning efficiency and a larger tuning range are obtained compared to Bragg mirror cavities. New behavior also emerges such as the possibility to tune the inner interface from being a node to an antinode of the standing wave electric field, and therefore the possibility to provide enhancement of emission of nanoemitters binding to the inner interfaces of the cavity, such as in microfluidic microassay systems.