A caudate sonnet is an expanded version of the sonnet. It consists of 14 lines in standard sonnet forms followed by a coda (Latin cauda meaning "tail", from which the name is derived).
The invention of the form is credited to Francesco Berni. According to the Princeton Encyclopedia of Poetry, the form is most frequently used for satire, such as the most prominent English instance, John Milton's "On the New Forcers of Conscience Under the Long Parliament."
Gerard Manley Hopkins used the form in a less satirical mood in his "That Nature is a Heraclitean Fire." The poem is one of many in which Hopkins experimented with variations on sonnet form. However, unlike the curtal sonnet, a Hopkins invention which is a 10½-line form with precisely the same proportions as a Petrarchan sonnet, his caudate sonnet is a full sonnet unmodified but with an extra six lines. Hopkins heightens the effect of the extension with an enjambment from the 14th line to the 15th.
Hopkins explored the possibility of such a coda in a series of letters exchanged with Robert Bridges, from whom he learned of the centrality of Milton's example in the form. Though the intent of his example is distinct from Milton's satirical use, the effect of the coda—to add stability to the poem's close—is comparable.