Sankt Goar line

In German linguistics, the Sankt Goar line, Das-Dat line (that), or the Was–Wat line (what) is an isogloss separating the dialects to the north, which have a /t/ in 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