English: Definition of full width at half maximum (FWHM) bandwidth for a resonance curve. The horizontal axis is frequency, the vertical axis is amplitude. The FWHM is the width in frequency of the shaded region between the points on the curve where the amplitude has dropped to half of the maximum amplitude. The variable labels in this drawing may have been accidentally switched, because the amplitude values on the vertical axis are labelled f_max, while the frequency values on the horizontal axis are labelled x₁ and x₂.