Phonetics is the systematic study of speech and the sounds of language. Traditionally phoneticians rely on careful listening and observation in order to describe speech sounds. In doing this, a phonetician refers to a classificatory framework for speech sounds which is based on how they are made and on aspects of the auditory impression they make. The best known such framework is that of the International Phonetic Association. Much of our knowledge of the sounds of the world's languages comes from this kind of description, which is still an important aspect of phonetics today.