This article presents a theory of the initiation and incrementation mechanisms whereby individual phonetic innovations become community-wide sound changes. The theory asserts that language learners are community-oriented and momentum-sensitive: they are community-oriented in that they acquire and obey a mental representation of the collective linguistic norm of their speech community, rejecting individual idiosyncrasies; they are momentum-sensitive in that their mental representation of the community norm includes an age vector encoding linguistic differences between age groups. The theory is shown to fulfil four critical desiderata: (i) it accounts for the sporadic and localized occurrence of community-wide sound change, (ii) it incorporates Ohala’s prediction of a lawful relationship between the strength of the phonetic biases driving individual innovation and the typological frequency of the corresponding sound changes, (iii) it explains how community-wide sound change advances by intergenerational incrementation producing adolescent peaks in apparent time, and (iv) it reliably generates monotonic—including sigmoid—diachronic trajectories. Moreover, the hypotheses of community orientation and sensitivity to momentum, combined with the mechanical effects of density of contact, suffice to explain several macroscopic phenomena in the propagation of sound change, including class stratification, the curvilinear pattern in change from below, and the existence of change reversals. During propagation, linguistic variants do acquire indexical value, and so social meaning, but this produces only small-scale attitudinal effects; it is not the force that drives the intergenerational incrementation of sound change.