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 and wat, from the dialects to the south, 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
