Which feature is common to all sonnets?

A key characteristic of all sonnets is their established rhyme scheme, which provides structure and musicality to the poem. Sonnets typically consist of 14 lines and follow specific patterns of rhyme, such as the Shakespearean (or English) sonnet, which has a rhyme scheme of ABABCDCDEFEFGG, or the Petrarchan (or Italian) sonnet, which usually follows an ABBAABBA pattern. This rhyme contributes to the formality and artistry of sonnets, allowing poets to convey their themes in a tightly controlled structure.

In contrast, the other options do not apply universally to all sonnets. Free verse poems do not adhere to a specific rhyme scheme, thus contrasting with the traditional structure of sonnets. While personal experiences may be a theme in some sonnets, it is not a defining feature that encompasses all sonnets, as they can also explore broader themes such as love, nature, and mortality. Furthermore, humor is not a requirement or common thread in sonnets, as they often express serious or reflective themes. Therefore, the requirement of a rhyme scheme is the distinctive trait that unites all sonnets.