Phonetic transcription represents the phonetic properties of an actual or potential utterance in a written form. Firstly, it is necessary to have an understanding of what the phonetic properties of speech are. It is the role of phonetic theory to provide that understanding by constructing a set of categories that can account for the phonetic structure of speech at both the segmental and suprasegmental levels; how far it does so is a measure of its adequacy as a theory. Secondly, a set of symbols is needed that stand for these categories. Also required is a set of conventions that tell the reader what the symbols stand for. A phonetic transcription, then, can be said to represent a piece of speech in terms of the categories denoted by the symbols. Machine-readable phonetic and prosodic notation systems can be implemented in electronic speech corpora, where multiple linguistic information tiers, such as text and phonetic transcriptions, are mapped to the speech signal. Such corpora are essential resources for automated speech recognition and speech synthesis.