(b Kaceřov, nr Loket, Bohemia, bap. 30 Sept 1685; d 21 March 1734). Bohemian composer, organist and choirmaster. He began his career as a singer at the Benedictine monasteries at Kladruby (1696) and St Mikuláš in the Old Town of Prague (from 1698). There he studied music with Prokop Smrkovský and Isidor Vavák (organ); at 16 he began to compose. During his studies of law and theology, Jacob was active from 1707 as assistant organist at the same monastic church and in 1711 he probably succeeded Vavák as choirmaster; some sources cite Jacob as choirmaster from 1705 and Vavák as organist, 1695–1713. As a member of the Benedictine order he accepted the name Guntherus and took his monastic vows on 1 November 1710. In 1719 he was appointed tutor of Countess Lažanská’s children in Manětín, where he stayed several times during the following years. On his numerous travels he visited Vienna and Lower Austria (1727), as well as some monasteries in Bohemia and Moravia (Rajhrad, Brno). His place of death is unknown.
Jacob was a renowned composer not only in his country, but also in Austria, Hungary and Bavaria; some of his works were offered in the catalogues (1736, 1748) of the Augsburg publisher J.J. Lotter. Of his large output (more than 100 compositions are listed in the inventories of the Osek and Rajhrad monasteries) only a minor part has survived.
Jacob is one of the most remarkable figures of the late Baroque in Bohemia. His point of departure as a composer was the works of his Prague contemporaries M. Wentzeli, F.L. Poppe and J.I.F. Vojta. Among his most important compositions are the masses from Acratismus, as well as several bass arias (Aria de ascensione, Ave regina, Cantata pentecostalis). His inclination for the Venetian polychoral style is attested by a mass for three choirs and three organs (1717; not extant). But survivals of the old concertato are generally superseded in Jacob's output by the instrumental idiom of the concerto style with wide-ranging themes, uniform rhythmic energy and running basses. Jacob wrote in both luxuriant counterpoint and continuo-homophony. His large works have multipartite structure with short instrumental introductions followed by contrasting solo and tutti sections without clearly designed architecture; he did not employ da capo forms. His treatment of words is expressive and emotional; his intensely dramatic attitude towards the text is most clearly evident in the recitatives. Despite occasional cross-relations, chromatic progressions and abrupt harmonic changes, Jacob's harmonic vocabulary is rather monotonous. He was at his best when writing for solo voices with obbligato solo instruments and organ continuo. Some traits of his musical language (melodic motifs of folk character, syncopated and dance rhythms, passages of parallel progressions in 6ths and 3rds) testify to Jacob's affinity to the Czech pre-Classical idiom.