This paper argues for a combined reading of Odes 1.3, the propempticon for Virgil, and Odes 1.22, the Lalage ode. The arrangement of the first book of Odes places 1.3 in structural relation to 1.22; and the appearance of scelus, ‘sin’, in the penultimate verse of 1.3 and the initial line of 1.22 activates a thematic affiliation that has gone unexamined in the scholarship. I take the poet's use of scelus as my starting point, analysing in what follows the ways in which Virgil and Horace are both positioned in the drama of 1.3 as practitioners of scelus of a kind that associates them with Prometheus, Daedalus and Hercules. Then I press the question of how such a depiction makes sense, given the ways in which these mythological exemplars are shown to be transgressors of natural boundaries. I then turn to 1.22 for an answer otherwise not forthcoming in 1.3, where scelus reappears in a poem about the power of song. I argue that the transgression Virgil and Horace practise in 1.3 is poetic composition itself—a sin Horace himself commits in the very odes that dramatise it, but whose staging extols the purity and integrity of a kind of singing that is prior to the concatenation of word, metre and music that go into the composing of any poetry. I see in these odes, then, a meditation on the ways in which poets cross back and forth between a boundary that separates originary song from the polished songs of poetry that it initiates—surely a topic that interested, and vexed, Horace for all of his career, and about which these odes have something fundamental to tell.