Inner mouth radical - the indexing component. Marks 句 as a unit of spoken language. The whole compound (wrap around mouth) gives the meaning: words from the mouth bundled into a single utterance. Hence the measure-word use for sentences (一句话 = one sentence).
Outer wrap component - pictures arms curving around to enclose. Marks 句 as something bounded: a sentence is a bundle of speech, wrapped into a complete unit. The same wrapping shape sits in 包 (wrap), 旬 (ten-day cycle), 勺 (ladle).
