What are the characteristics of an elliptical filter?

Elliptical filters, also known as Cauer filters, are designed to produce the sharpest possible transition between the passband and the stopband for a given number of components. To achieve this extremely steep roll-off, elliptical filters allow for ripple in both the passband and the stopband. The ripple in the stopband creates specific frequencies with near-infinite attenuation, which appear as deep 'notches' in the frequency response. This makes elliptical filters ideal for applications where you must strictly separate two closely spaced frequencies. Distractors mentioning a flat response or gradual roll-off describe Butterworth filters, not elliptical ones.