What was the ‘alt’ in alt-right, alt-lite, and alt-left? Tracing these controversial terms’ development over the 2010s, this article interrogates the construction, meaning, and utility of the ‘alt’ modifier in US politics. Contextualizing the emergence of ‘alt’ among wider debates about how to spatially conceptualize anti-system politics beyond the left-right spectrum, it argues that ‘alt’ was used to name a tendency that moved beyond traditional ideological questions about what society should look like, and instead signified a preference about how politics should be done: namely, a rejection of the mainstream norms of political conduct, and an embrace of vulgarity, incivility and an extremely adversarial approach to political opposition. It shows how a popular-cultural understanding of ‘alternative’ was used to give this approach a countercultural sheen, and maps out the cleavages between groups that the alt/mainstream binary opened up. Finally, it considers the legacy of the ‘alt’ modifier in contemporary politics.