The phoneme can be roughly defined as a minimal unit of sound that can be used to distinguish words in a language. The question of how to theoretically define the phoneme dominated linguistics from the late 19th century until about 1960. In 1959, Morris Halle first published his famous argument against the “classical phoneme” in his monograph The Sound Pattern of Russian. Halle and Chomsky reiterated that argument in The Sound Pattern of English, which had been widely circulated in the 1960s in various manuscript forms before it was published in 1968. The argument, now familiar to most linguists, had to do with an asymmetry in the Russian phoneme inventory. Halle had noticed that Russian regressive voice assimilation produced both phonemic and allophonic outputs. That is, it could produce a phonemic output /z/ from an underlying /s/ or an allophonic variant [ɣ] from an underlying /x/. If one were to posit a significant level of classical phonemic representation in a phonological derivation, then it would be necessary to apply regressive assimilation twice–above and below the level of phonemic representation. Hence, classical phonemic representation had to be ruled out as a significant level of phonological representation. This argument basically signaled the death of the phoneme in early generative phonological theory. As a consequence, the input to phonological derivations became the level of systematic phonemics, which captured generalizations about morphophonological alternations, a considerably more abstract level of representation than in classical phonemic theories.