Sankt Goar line

From Freepedia

In German linguistics, the Sankt Goar Line is an isogloss separating the dialects to the north, which have a /t/ is the words dat "that" and wat "what", from the dialects to the south (including standard German), which have an /s/: das, was. The line runs from east to west and crosses the river Rhine at the town of Sankt Goar.

See: High German consonant shift