Caesura is a break or pause in a line of poetry, dictated by the natural rhythm of the language and/or enforced by punctuation.
A line may have more than one caesura, or none at all. If Caesura is near the beginning of the line, it is called the initial caesura; near the middle, medial; near the end, terminal. The commonest is medial. An accented (or masculine) caesura follows an accented syllable, and an unaccented (or feminine) caesura an unaccented syllable.
In OE verse the caesura was used rather monotonously to indicate the half-line:
Đa waes on burgum || Beowulf Scyldinga
leof leodcyning || longe þrage.
Another famous example is from William Langland’s Piers Plowman
Loue is leche of lyf || and nexte owre lorde selue,
And also þe graith gate || þat goth in-to heuene.
The development of the iambic pentameter in Geoffrey Chaucer’s hands produced much more subtle varieties:
With him ther was his sone, || a yong Squier
A lovyere || and a lusty bacheler,
With lokkes crulle || as they were leyd in presse. (Prologue to the Canterbury Tales)
Blank verse allowed an even wider range in the preservation of speech rhythms, as these lines from Shakespeare’s King Henry VIII suggest:
I have ventur’d
Like little wanton boys || that swim on bladdrs,
This many summers || in a sea of glory;
But far beyond my depth. || My high-blown pride
At length broke under me, || and now has left me,
Weary and old with service, || to the mercy
Of a rude stream || that must for ever hide me.
In more modern verse there is a great deal of variation in the placing of the caesura, as can be seen in the work of outstanding innovators like Gerard Manley Hopkins, W. B. Yeats, Ezra Pound, and T. S. Eliot. One famous example is from Yeats:
That is no country for old men. || The young
In one another’s arms, || birds in the trees
– Those dying generations – || at their song,
The salmon-falls, || the mackerel-crowded seas,
Fish, flesh, or fowl, || commend all summer long
Whatever is begotten, || born, || and dies.
Caught in that sensual music || all neglect
Monuments of unageing intellect. (Sailing to Byzantium)
It will be noticed that the last line has no caesura. It can be seen from these few examples that the caesura is used, basically, in two contrary ways: (a) to emphasize Formality and to stylize; and (b) to slacken the stiffness and tension of formal metrical patterns.
Also read; What is Novel of sensation and its examples