When a segment of a signal is correlated with itself, with the phase progressively changed from 0 (trivially maximum correlation) to 
, the point at which the next closest maximum correlation occurs may be taken to define the fundamental period of the signal (whose inverse is the fundamental frequency, or 
. The segment to compared should be at least as long as the longest period (the lowest frequency) to be expected. The problem is more complex than this approach makes it appear, since correlation of the period of the fundamental frequency may have a lower value in some contexts in practice than, for instance, the correlation with the period of the second harmonic.