Browning's use of enjambment makes the poem sound conversational. It tends to obscure where one line ends and another begins, which subtly deemphasizes the meter, rhyme scheme, and number of feet in the line.
This poem uses predominantly enjambed lines throughout in part to reflect the fact that it is a dramatic monologue, and therefore is understood as being spoken to a listener. Again, the free-flowing lines suggest a conversational tone. They also suggest that the duke likes to hear himself talk, as his speech spills over from one line to the next offering no chance for his guest to voice any interruptions. Enjambment thus allows the duke to control the conversation and reflects his need to dominate and manipulate those around him.
One of the most striking examples of enjambment occurs towards the end of the poem, in lines 31-39:
She thanked men—good! but thanked
Somehow—I know not how—as if she ranked
My gift of a nine-hundred-years-old name
With anybody’s gift. Who’d stoop to blame
This sort of trifling? Even had you skill
In speech—which I have not—to make your will
Quite clear to such an one, and say, “Just this
Or that in you disgusts me; here you miss,
Or there exceed the mark"
Because of the seven consecutive enjambed lines, it is nearly impossible to tell where one line ends and another begins when the passage is read out loud. The words topple over the boundaries of the line, as though the duke is so horrified by the duchess’s actions and by the idea of confronting her that he can no longer make clear, linear sentences. It might seem as though the duke truly does lack the “skill / In speech” to reprimand her.
However, the duke had mentioned earlier that he has spoken to others about his wife’s behavior before, so this conversation with the emissary is not the first on the subject, and his love of controlling conversations with props (like the painting, chair, and curtains) further suggests that he plans out all details, no matter how small.
As a result, the enjambment that seems to show the duke losing control over the conversation is arguably evidence that even his loss of control is all an act, and that he has scripted even his supposedly unscripted moments. Essentially, then, Browning uses enjambment to show the duke’s obsession with controlling how others view him and to show that the duke is fundamentally untrustworthy.