Swinburne gives life's two largest facts a family tree and a throne. Birth and death are not enemies, and they are not simply the start and finish of a road. They are "twin-sister and twin-brother," born together and sharing power over "all things that draw breath." The poem's central idea is that the two cannot be pulled apart. Where one reigns, the other reigns beside it, and the third stanza extends the same logic to joy and sorrow, which turn out to be just as hard to tell apart.
The imagery is a mix of family and monarchy. The twins are siblings, but they also "Reign," and each is "brow-bound with flowers diverse of wreath," a crowned head with a garland. Flowers are a clever choice here. They suggest festivals and weddings, but also funerals and graves, so the same wreath could belong to either twin. Their parents are "Heaven" and "earth," whom they "hail" as father and mother. The twins are therefore rooted in the whole cosmos, not just in human life. Notice too how the poem slides into "Night and day" in its second line. Birth and death start to feel less like events and more like a rhythm of the world, a turning that never stops. The phrase "while time keeps friends with one another" suggests that time's job is to keep the two on good terms.
The middle stanza is only three lines long, and its last phrase is wonderfully open. "Faithful found above them and beneath" could describe the twins, who are reliable wherever one looks, or heaven and earth, which stay true to their children. The poem doesn't say, and that ambiguity adds to its calm. Nothing here promises an afterlife or complains about fate. The tone is closer to that of someone watching the tide.
The final stanza turns to emotion. "Smiles may lighten tears, and tears may smother / Smiles" is a neat chiasmus, a crossing of word order that makes each feeling take its turn on top. "Lighten" can mean to relieve or to brighten, and "smother" is blunt and physical. Then comes the odd grammar of "Joy nor sorrow knows not from each other." Read strictly, the double negative muddles the sense, but in the older, emphatic style it means that joy and sorrow cannot tell themselves apart. The tangled line suits the idea, since it is a sentence in which the two sides blur together.
All of this sits inside a very controlled shape. The poem is a roundel, a form of French origin that Swinburne adapted for English. Its refrain, "Birth and death," echoes the poem's opening words and returns at the end of the first and last stanzas, so the poem begins and ends on its subject like a ring. It uses only two rhyme sounds, the -other group (brother, another, mother, smother, other) and the -eath group (breath, death, wreath, beneath, saith). The first group are double rhymes, with a soft falling second syllable, while the second are single and land with a thud. That pairing mirrors the twin theme in sound. The -eath words don't match perfectly, since "breath" and "death" have a short vowel and "wreath" and "beneath" a long one, but they hold together on the page. The long, rocking lines, with their trochaic stress on "BIRTH and DEATH," are cut off by the two-beat refrain, which hits like a bell.
The poem matters for how much it does in so little space. In eleven lines it presents an old subject without sentimentality or fear, and it does so through sound as much as argument. For a student reader it is a good example of how form can carry meaning. The repeating refrain and the paired rhymes enact the idea that these opposites belong together, and the poem finishes where it started, as if it were also turning in a circle.