The Synod of Mantaille was held in Mantaille, in the southwestern french region of Provence

Provence

Provence ; Provençal: Provença in classical norm or Prouvènço in Mistralian norm) is a region of south eastern France on the Mediterranean adjacent to Italy. It is part of the administrative région of Provence-Alpes-Côte d'Azur...

, on 15 October 879 by the bishops and nobles of the region around the rivers Rhône

Rhône River

The Rhone is one of the major rivers of Europe, rising in Switzerland and running from there through southeastern France. At Arles, near its mouth on the Mediterranean Sea, the river divides into two branches, known as the Great Rhone and the Little Rhone...

 and Saône

Saône

The Saône is a river of eastern France. It is a right tributary of the River Rhône. Rising at Vioménil in the Vosges department, it joins the Rhône in Lyon....

. They elected Boso of Provence

Boso of Provence

Boso was a Frankish nobleman from the Bosonid-family, who was related to the Carolingian dynasty, who rose to become King of Provence ....

, Count of Vienne

Count of Vienne

The Count of Vienne was the ruler of the Viennois, with his seat at Vienne, during the period of the Carolingian Empire and after until 1030, when the county of Vienne was granted to the Archdiocese of Vienne....

, as successor to king Louis II of France, dead since April. The synod marks the first occurrence of a "free election" among the Franks

Franks

The Franks were a confederation of Germanic tribes first attested in the third century AD as living north and east of the Lower Rhine River. From the third to fifth centuries some Franks raided Roman territory while other Franks joined the Roman troops in Gaul. Only the Salian Franks formed a...

, without regard to royal descent, inspired by the principles of ecclesiastical elections.