FIG. 16.  The locus theory, illustrated in the right panel for the consonant /d/ before a series of vowels having the same F1, states that the second formant transition appears to originate from an invisible locus at 1800 Hz, after Delattre et al. (1955). If the second formant onset frequency (hub) is fixed at 1800 Hz, left panel, several different consonants are heard.