WILLIAM (1227-1256), king of the Romans and count of Holland, was the son of Count Floris IV. and his wife Matilda, daughter of Henry, duke of Brabant. He was about six years of age at his father's death, but his long minority, under the guardian ship of his two paternal uncles, was peaceful. In 1247 William allowed Pope Innocent IV. to proclaim him king of the Romans in opposition to. the excommunicated Frederick II., and having taken Aix-la-Chapelle, was crowned there on All Saints' Day, 1248. He
thus became the recognized head of the Guelph party, but even after Frederick's death he had gained few adherents, when he was killed on Jan. 28, 1256. He was more successful in asserting the rights of John of Avennes, who had married his sister Aleidis, to the county of Hainaut against John's mother, Margaret, whom he defeated decisively at West Kappel in 1253.
See A. Ulrich, Geschichte des romischen Konigs, Wilhelm von Holland (Hanover, 1882).