Language is the immediate actuality of thought and the most important means of communication. It exists in two main speech forms: oral and written. In oral speech the substance is phonic In written speech the substance is graphic The phonetic system of a language is a set of phonetic units arranged in an orderly way to replace each other in a given framework. It contains two systems: Segmental units are elementary sounds, vowels and consonants Prosodic units are syllables, rhythmic units, and intonation groups, utterances