It starts on the north side of Alpstein massif in Appenzell Innerrhoden, and joins River Thur near the town of Bischofszell in Thurgau.

The Sitter begins in the canton of Appenzell Innerrhoden, in Weissbad at a height of 810 metres (2,660 ft) above sea level, at the confluence of Weissbach and Schwendibach.

Although it is shorter than the Weissbach, it has a larger catchment basin and a stronger average discharge (MQ), making it the hydrological main branch of the Sitter river system.

It has a catchment area of 26.52 square kilometres (10.24 sq mi), an average discharge of 1.35 m3/s (48 cu ft/s) and begins north of the Böhlhütte at a height of 1,004.7 metres (3,296 ft) above sea level, at the convergence of the Seckbach and Fallbach.

[2] The 339.94 square kilometres (131.25 sq mi) large catchment area of the sitter stretches from the Appenzell Alps to the Swiss Plateau und is drained through it via the Thur and the Rhine to the North Sea.

In 1949, Swiss Post issued a five-cent postal stamp depicting the Sitter bridges near St.