As a theory of phonological representation, autosegmental phonology developed a formal account of ideas that had been sketched in earlier work by several linguists, notably Bernard Bloch (1948), Charles Hockett (1955) and J. R. Firth (1948). According to such a view, phonological representations consist of more than one linear sequence of segments; each linear sequence constitutes a separate tier. The co-registration of elements (or "autosegments") on one tier with those on another is represented by association lines. There is a close relationship between analysis of segments into distinctive features]] and an autosegmental analysis; each feature in a language appears on exactly one tier.