This phenomenon occurs because voiced fricatives have developed from lenition of plosives or fortition of approximants. This phenomenon of unpaired voiced fricatives is scattered throughout the world, but is confined to nonsibilant fricatives with the exception of a couple of languages that have but lack . (Relatedly, several languages have the voiced affricate but lack , and vice versa.) The fricatives that occur most often without a voiceless counterpart are – in order of ratio of unpaired occurrences to total occurrences – , , , and .