The spectrum of the logarithm of a frequency spectrum; the term arises from a letter reversal in the word spectrum. A cepstrum is often used in speech analysis for fundamental frequency and formant analysis. The X-axis has the dimension of time and is referred to as quefrency (from letter reversal in ‘frequency’), and regularly spaced peaks in the cepstrum are known as rahmonics (from letter reversal in ‘harmonics’). Low-frequency components, such as the fundamental frequency in speech, have a high quefrency, and high-frequency components, such as formant peaks, have a low quefrency, so they can be separated by means of a lifter (from letter reversal in ‘filter’).