Unlike letters, IPA characters represent individual sounds. This Appendix cannot replace a course or book in phonetics, but for the benefit of readers with little or no training in phonetics (or those who need a reminder) here is an attempt at defining the sound symbols by explaining what happens in the mouth in the articulation of the respective sounds. (Further phonetic terms are explained in the Glossary.)
Fundamentally and also in terms of articulatory processes, it makes sense to distinguish vowels from consonants.
The difference between how individual vowels sound (their “quality”) is achieved by
different degrees of opening the mouth, which correlate with a higher or lower position of the hump which the tongue forms, and
by moving this hump (the “back” of the tongue) back and forth.
So it is defined best by indicating the position of the back of the tongue on a two-dimensional plane insider the oral cavity, with the two dimensions being defined as
(For the “high–low” dimension the terms “close–open” are also used.)
The shape of this two-dimensional space, within which the hump of the tongue can move, roughly has the shape of a trapezium, so the quality of any individual vowel (symbol) is conventionally indicated by showing its position in this “vowel trapezium.”