A caesura, plural caesuras or caesurae, is a type of pause in a line of poetry. Such a pause can occur either towards the beginning, in the middle, or towards the end; however, a caesura cannot occur right at the end of a line. A caesura is actually very similar to what we call an end-stopped line; however, just like the name end-stopped line implies, end-stopped lines create pauses at the very end of lines by ending a thought at the end of a line, while caesurae create pauses somewhere within the actual words of the line themselves. Caesuras can be created through punctuation, such as commas, semicolons, or periods in the middle of lines, or caesurae can simply be created through the natural rhythmic or grammatical structure of the words themselves. We can mark caesuras using double back slashes: \\. The Encyclopedia of Britannica gives us the following example of caesuras being created through the use of commas found in Shakespeare:
This blessed plot,\\ this earth, \\this realm,\\ this England, ...
We can find a similar example in the very first line of "Old Ironsides" by Oliver Wendell Homles: "Ay,\\ tear her tattered ensign ...." Observe how the comma plays a crucial role in grammar to mark the interjection "Ay," which means yes? There is even another similar situation in the first line of the second stanza and more after that.
However, recognizing caesuras can be more challenging when there are no punctuation marks to help us out. For pauses without punctuation, we simply look at pauses created by wording. Most of Holmes's pauses are actually created at the end of lines; however, if we look at lines like, "Should sink beneath the wave," see that we have two consonants following each other in the phrase "sink beneath." It's impossible, when speaking, to move from consonant straight to another consonant without creating a pause in breath. Such pauses can also be heard in the meter; therefore, the third stanza contains another example of the following caesura: "Should sink // beneath the wave."