In the theories of pitch perception now widely supported, pitch is regarded as a unitary attribute of auditory experience. There is good evidence, however, that there are actually two pitch-like attributes, and it is reasonable to suppose that the duplexity of pitch is a reflection of duplexity in the auditory process. The first step in the process is analysis in frequency, performed by the cochlea, which distributes stimulus components of various frequencies to spatially separated channels. The second step, according to the scheme postulated here, is autocorrelational analysis, performed by the neural part of the auditory system, of the signal in each frequency channel. The basic operations of autocorrelational analysis are delay, multiplication, and integration. The nervous system is nicely set up to perform these operations. A chain of neurons makes an excellent delay line. The spatial aspect of synaptic summation provides something very close to multiplication. And the temporal aspect of synaptic summation is essentially running integration. The duplex theory suggests, therefore, that neural circuits following the autocorrelation model supplement the cochlear frequency analysis. The postulated neural autocorrelator of course does not compute autocorrelation functions of the acoustic stimulus: it operates upon afferent neural signals. Because the markedly non-linear process of neural excitation intervenes between the stimulus and the autocorrelation, the latter gives rise in certain instances to pitches that are not readily explained if the relatively linear cochlear analysis is considered to be the only one. “The case of the missing fundamental” and Schouten's residue effect, for example, are readily accounted for by the duplex theory. In addition, the theory provides a rational basis for the octave relation and for the consonance of other simple harmonic relations.