This is a study of the competition of linguistic systems within the speech community, tracing the opposition of distinct phonological configurations. Among younger Philadelphians oriented to higher education, the traditional short-a system is giving way to the nasal system, in which all prenasal vowels are tense. We present evidence that this shift occurs systematically in both the lax and tense allophones. The degree of conformity to each system is measured for 106 subjects by the Pillai statistic; community-wide patterns are revealed by regression modeling and bimodality diagrams. Adherence to the traditional system is greatest for White speakers who graduated from Catholic high schools, while the nasal system is strongest for graduates from elite schools with special admission requirements. African American students show no orientation to the traditional system, but parallel White students in the shift to the nasal system. The histories of students through the educational system show that the speaker's short-a system is largely determined during their middle and high school years, before entering college. Special admission interviews play a special role in this process, as brief speech events that strongly determine the person's linguistic environment for the years that follow.