Caesura
The main cut dividing a line into two hemistichs — the classical line's rhythmic anchor.
The caesura is a long line's main cut, the one splitting it into hemistichs — after the sixth syllable in the classical alexandrine. It isn't a mere reading pause: classical prosody codifies the spot (no unelided mute e at the caesura, mandatory word ending, ideally a support of meaning).
Working it means turning rhythm into meaning: a clean caesura gives balance and majesty; a jostled one makes the line limp on purpose. Hugo boasted of having "dislocated that great ninny the alexandrine" — the dislocation happens first at the caesura. In free verse, the caesura survives as a mobile internal cut, moved at will to sculpt the sentence.
Example
"To err is human; // to forgive, divine."