What's Montague? || It is nor hand nor foot
--William Shakespeare, Romeo and Juliet (II.ii.40)This line is in blank verse. But how is this line different? There is a pause mid-verse here, between Montague and It separating an iamb made up of two syllables.
This natural pause or the break mid-line is called a Caesura. But a Caesura need not only occur in a blank verse. In an iambic pentameter it occurs between the first and second syllable of an iamb like in this example. Further, in this example because the pause follows an unstressed syllable, this is a feminine caesura.
However, this metrical technique is employed in other verse structures as well and is not typical of blank verse. It is intended to bring about a dramatic pause. This, originally, is a feature of the classical Greek and Latin poetry, and eventually, a frequent feature of Anglo Saxon poetry.
The woods are lovely, || dark and deep
--(Robert Frost, Stopping by Woods on a Snowy Evening)Here, too, the caesura follows an unstressed syllable, making this a feminine caesura but in an iambic tetrameter.
To be, || or not to be; || that is the question
--William Shakespeare, Hamlet [III.i.57]In this rather famous first line of the Hamlet soliloquy there are two caesuras. And both the caesuras are masculine. These are initial and medial caesuras. The one occurring in the beginning is an initial caesura and as the name would suggest the one in the middle a medial caesura. Further, there is a terminal caesura, absent here, in this example, but one that occurs just before the end of a line.
In the same example, the second masculine caesura is followed by a stressed syllable making it a trochee and not the typical iamb. These along with the caesuras are all metrical techniques used to break the rhythm of a regular speech pattern for dramatic effect.