"Death, be not proud" is a sonnet in two stanzas. It closely resembles the structure of a Petrarchan sonnet, named for the Italian poet Petrarch, but Donne makes some unique adjustments to his rhyme scheme.

A Petrarchan sonnet's first stanza, known as an "octave," is eight lines long. Traditionally, the rhyme scheme for an octave is abbaabba. In the poem "Death, be not proud," Donne's octave follows the rules of this traditional form.

The second stanza, known as the "sestet" for its six lines, is where Donne changes the formula somewhat. Petrarchan form dictates that the rhyme scheme for the sestet should be cdcdcd or cdecde, but Donne's work finishes with a pattern of his own: cddcaa or cddcae, depending on whether the speaker's inflection treats the -ly at the end of "eternally" and the word die as though they rhyme.

In the reference links, I've included a helpful page from the Poetry Foundation that elaborates on sonnet structure and form.