Short answers, pulled from the story.
The Golden Horns of Gallehus discovered in Denmark and likely dating to the 4th century bear a Runic inscription that serves as one of the earliest surviving examples of alliterative verse. The text reads ek hlewagasti r holtija r || horna tawid o which translates to I Hlewagastir son of Holt made the horn.
A long line is divided into two half-lines known as verses or hemistiches with the first called the a-verse and the second the b-verse. Each verse usually has two heavily stressed syllables referred to as lifts or beats while other less heavily stressed syllables are called dips.
Starting in the mid-14th century alliterative verse became popular in the English North the West Midlands and a little later in Scotland. William Langland's Piers Plowman was written between c. 1370 and 1390 during this period of popularity.
J.R.R. Tolkien used alliterative verse extensively in both translations and original poetry with some poems embedded in the text of his fantasy novel The Lord of the Rings. His longest modern English works include an alliterative verse play The Homecoming of Beorhtnoth Beorhthelm's Son published in 1953 and his 2276-line The Lay of the Children of Húrin published in 1985.
From 19th century poets like Jonas Halgrimsson to 21st-century poets like Valdimar Tómasson alliteration has remained a prominent feature of modern Icelandic literature though contemporary Icelandic poets vary in their adherence to traditional forms. Alliterative verse appears to have been the dominant poetic tradition in Iceland until well after World War II.