Phonology no longer regards the phenomena it studies as a linear sequence of phonemes and a collection of processes which describe the changes which occur in this sequence. Goldsmith proposes to represent phonological phenomena as a collection of parallel tiers with individual segments which represent certain features of speech. Various feature systems have been proposed. Depending on the nature of the study already a few can explain phenomena which are difficult to explain otherwise. An example would be one tier representing the conventional segments (in the sense of phonemes) and an additional tier representing tone. (see tone language)