Sankt Goar line

In German linguistics, the Sankt Goar line, das–dat line, or was–wat line is an isogloss separating the dialects to the north, which have a t in the words dat (English that) and wat (English what), from the dialects to the south (including standard German), which have an s: das, was. The line runs from North-East to South-West and crosses the river Rhine at the town of Sankt Goar.