O Rose, thou art sick! The invisible worm That flies in the night,
Why it works
Blake's poem consists of two loose tercets, and the opening one illustrates how the form can function without rigid meter or complete rhyme. The three lines shift from a direct address to an ambiguous, unresolved image — the worm is mentioned but not yet clarified. The tercet breaks after 'night,' creating a pause before the second stanza reveals the consequence. This gap between the stanzas serves a dramatic purpose and arises solely because Blake opted for three-line units instead of quatrains.