on the diagram, a phenomenon that is known as vowel dispersion. Most languages have a vowel system with three extreme points, forming a vowel triangle. Only 10% of languages, including English, have a vowel system with four extremes. Such a diagram is called a vowel quadrilateral or a vowel trapezium.

Alternatively, these shape-based terms are used to distinguish the method of analysis used for vowel diagrams: vowel triangles for diagrams typically based on § acoustics, and vowel quadrilaterals for diagrams