Sosau was formerly part of the German Empire. In the German Empire, the place was called Sosau.
The place is now called Zásada u Rašovic (Kadaň-Zásada u Raš.) and belongs to Czech Republic.
| Historical place name | Country | Administration | Time |
|---|---|---|---|
| Sosau | Austro-Hungary | Kaaden | before the Treaty of Saint-Germain |
| Zásada | Austro-Hungary | Kaaden | before the Treaty of Saint-Germain |
| Zásada | Czechoslovakia | Kadaň | after the Treaty of Saint-Germain |
| Sosau | Czechoslovakia | Kadaň | after the Treaty of Saint-Germain |
| Sosau | German Empire | Kaaden | 1938 |
| Zásada | Czechoslovakia | Kadaň | 1945 |
| Zásada u Rašovic (Kadaň-Zásada u Raš.) | Czechoslovakia | Chomutov | 1992 |
| Zásada u Rašovic (Kadaň-Zásada u Raš.) | Czech Republic | Chomutov | 1993 |