MERSEN (MEERSSEN), TREATY OF  , a treaty concluded on the 8th of August 87o at Mersen, in Holland, between Charles the Bald and his half-brother, Louis the German, by which the kingdom of their nephew Lothair II . (d . 869) was divided between them . Charles received a portion of the kingdom of Lothair afterwards called Lorraine, extending from the mouths of the Rhine to Toul, together with the town of Besancon, the Lyonnais, the Viennais, the Vivarais, and 'the Uzege, i.e. the lands acquired by Lothair II. in 863 at the death of his brother Charles of Provence; while Louis had the cities of Cologne, Trier and Metz, together with Alsace, the Escuens, and the Varais, i.e. the greater part of the diocese of Besancon . The boundary between the two realms was marked approximately by the valleys of the Meuse and Moselle and by the Jura . Great importance has been attached to the determination of this frontier by some historians, who consider that it coincided with the dividing line between the Teutonic and Romance races and languages; but nothing is known of the bases upon which the negotiations were effected, and the situation created by this treaty came to an end in 879 .