English: The physical meaning of the Lorentzian finesse ${\displaystyle {\mathcal {F}}_{c}}$ of a Fabry-Pérot resonator. ^[1] Displayed is the situation for ${\displaystyle R_{1}=R_{2}\approx 4.32\%}$, at which ${\displaystyle \Delta \nu _{c}=\Delta \nu _{FSR}}$ and ${\displaystyle {\mathcal {F}}_{c}=1}$, i.e., two adjacent Lorentzian lines (dashed colored lines, only 5 lines are shown for clarity) cross at half maximum (solid black line) and the Taylor criterion for spectrally resolving two peaks in the resulting Airy distribution (solid purple line) is reached.