Keats’s central theme in “On the Sonnet” is the poet’s successful handling of the demands of form. He wrote in many forms: songs, romances, epistolary poems, epics, hymns, ballads, and odes. He also composed more than sixty sonnets. Here he had inherited two different traditions. The first was the Italian form of Dante and Petrarch, which consisted of an octave with an abba rhyme scheme followed by a sestet, which allowed for a variety of possible rhyme schemes: cde, cde; or cdc, cdc; or cd, cd, cd. This Italian form required an eight-line development, allowing only two rhyme patterns and ending in a complete stop; this was then followed by the slightly more flexible sestet. Overall, this form of the sonnet prescribed four, or perhaps five, rhyme sounds, and no more.

The sonnet form in England established a new rhyme pattern, ultimately labeled “Shakespearean”: abab, cdcd, efef, gg. Here, instead of an 8/6 division, there is a 4/4/4/2 pattern; also, instead of four or five rhyme sounds, there are seven. In May, 1819, Keats wrote to his brother George expressing dissatisfaction over the “pounding rhymes” of the Italian form and over the hurried snap of the concluding Shakespearean couplet. He then included “On the Sonnet,” which illustrated his continuing experiments with the form. This sonnet’s rhyme scheme is completely irregular: abc, abd, cab, cde, de. It follows neither the Petrarchan octave nor the Shakespearean quatrains and concluding couplet.

Keats’s sonnet addresses his theme of the necessary but troublesome traditional stanza patterns and their rhyme schemes. The poet freely experiments with both, leading directly to the triumphant odes of Keats’s annus mirabilis. These odes of 1819—“To a Nightingale,” “Ode on a Grecian Urn,” “Ode on Melancholy,” and “Ode on Indolence”—display the fruit of Keats’s sonnet experimentation, for this experimentation led directly to the new lyric form used in the great odes. Here Keats conserves some of the elements of both sonnet forms: the quatrain of the English form (abab) and the sestet of the Petrarchan sonnet (cde, cde). However, he organically extends these forms, makes them more malleable, “more interwoven and complete/ To fit the naked foot of poesy.” In his odes, he successfully prunes out the “dead leaves” in the bay-wreath crown, avoiding monotony and gaining new freedom. Thus he develops the perfect genre he had been seeking in “On the Sonnet.”