 When one vowel sound moves into another vowel's associated state, the result may be a merger of the two vowels or a reactive movement in a neighboring vowel. Such a change can spark a series of vowel movements that may result in multiple vowels receiving new places of articulation and tongue height. These natural movements can be seen in a sound system shift known as the northern city shift, which received its name from its association with the urban centers around the Great Lakes, including Chicago, Detroit, Milwaukee, Cleveland, and Buffalo. In the northern city shift, six different vowels are affected. The first change in the shift was the raising and tensing of short R. This left an empty space in the low front position. The low back vowel also shifts, becoming more like the vowel of cat and some speakers at the same time leading to a lowering of R, which then triggers a movement of R so that the vowel in a word like bus may sound closer to boss. Finally, the vowel in bit is backed and lowered so that its pronunciation nears the word bed. The connections among these changes are naturally determined and the whole shift resembles a clockwise rotation of the vowels in the mouth.