A hemistich is half of a line in a poem, with a pause before and after it that creates a complete unit of rhythm or verse. In Greek and Latin plays, hemistichs were often used, especially in tragedies where characters spoke in short, quick lines to show excitement or urgency. In the Roman poem The Aeneid, the poet Virgil used hemistichs to show when characters were too upset or hurt to speak in full lines.

During the neo-classical period, some poets, like John Dryden, avoided using hemistichs. However, in Germanic poetry, hemistichs were a key part of verse structure. In Old English and Old Norse poetry, each line was split into two parts, called "a-verse" and "b-verse," with a strong pause between them. In the Old English poem Beowulf, there are five main types of hemistichs, some used only at the beginning of lines and others only at the end. Middle English poetry also used hemistichs, with poets like the Pearl Poet and Layamon following rules to organize their verses using both rhythm and alliteration.

In Arabic and Persian poetry, each line is usually made of two equal hemistichs, forming a pair of lines. In some styles, like mathnawi or masnavi, the two parts of each line follow a rhyme pattern such as AA BB CC DD. In other styles, like ruba'i, qasida, or ghazal, the rhyme pattern is AA BA CA DA, with the second part of each pair of lines sharing the same rhyme.