Nyquist frequency

In this example, is the sampling rate, and is the corresponding Nyquist frequency. The black dot plotted at represents the amplitude and frequency of a sinusoidal function whose frequency is 60% of the sample rate. The other three dots indicate the frequencies and amplitudes of three other sinusoids that would produce the same set of samples as the actual sinusoid that was sampled. Undersampling of the sinusoid at is what allows there to be a lower-frequency alias. If the true frequency were , there would still be aliases at 0.6, 1.4, 1.6, etc. The red lines depict the paths (loci) of the 4 dots if we were to adjust the frequency and amplitude of the sinusoid along the solid red segment (between    and  ).  No matter what function we choose to change the amplitude vs frequency, the graph will exhibit symmetry between 0 and    This symmetry is commonly referred to as folding, and another name for    (the Nyquist frequency) is folding frequency. ==Other meanings==

Early uses of the term Nyquist frequency, such as those cited above, are all consistent with the definition presented in this article. Some later publications, including some respectable textbooks, call twice the signal bandwidth the Nyquist frequency; this is a distinctly minority usage, and the frequency at twice the signal bandwidth is otherwise commonly referred to as the Nyquist rate. ==Notes==