Theiss, the longest river of Hungary and largest of the affluents of the Danube; is formed in East Hungary by the confluence of the White Theiss and the Black Theiss, both springing from south-western slopes of the Carpathians; after a great sweep to the NW. bends round to the S., and flows steadily southward through the centre of Hungary until it joins the Danube 20 m. above Belgrade, after a course of 750 m.; with its greater tributaries, the Maros and the Bodrog, it forms a splendid means of internal commerce.