One topic that has begun to receive much attention from scholars in recent decades has been the emergence of a vowel-based chain shift in California. This image on the right illustrates the California vowel shift. The vowel space of the image is a cross-section (as if looking at the interior of a mouth from a side profile perspective); it is a rough approximation of the space in a human mouth where the tongue is located in articulating certain vowel sounds (the left is the front of the mouth closer to the teeth, the right side of the chart being the back of the mouth). As with other vowel shifts, several vowels may be seen moving in a chain shift around the mouth. As one vowel encroaches upon the space of another, the adjacent vowel in turn experiences a movement in order to maximize phonemic differentiation.