The inventory of sounds in a given language tends to be symmetrical. Sounds tend to be spread out evenly throughout the articulatory possibilities of a language.

(Spanish): Symmetrical vowel system

Here is an example of a symmetrical vowel system:

Spanish is an example of a symmetrical five vowel system. The five vowel system is the most common among languages of the world.

Asymmetrical vowel system

Here is an example of an asymmetrical vowel system that is not known to exist: