It has been argued (Tarone 1979) that the most systematic speech style of interlanguage is the vernacular, which is the speech style in which the learner pays the least attention to speech form. In this paper, the notions of “systematicity” and “attention” are developed in more detail. It is argued that attention is the variable which causes style-shifting along an interlanguage continuum of styles. These styles range from the superordinate style (in which the most attention is paid to language form) to the vernacular style (in which the least attention is paid to language form). It is further argued that the interlanguage vernacular is systematic in that it is describable and predictable by a set of rules, and that the vernacular is the most systematic style in that it is the style which is least permeable to invasion from other rule systems.