As we have seen, in any investigation of speech, it is on the physiological and acoustic levels that most information is available to us. But in any articulation, as revealed by MRI, an utterance consists of apparently continuous movements by a very large number of organs; it is almost impossible to say, simply from a video of the speech organs at work, how many speech sounds have been uttered. A display of acoustic information is slightly easier to handle (see Fig. 3), but even here it is not always possible to delimit exactly the beginning and end of sound segments because of the way in which many sounds merge into one another. Moreover, even if we were able to delimit and identify certain sounds, it would not follow that all the individual units would fit into a useful linguistic description of the language being investigated. Thus, the word tot is frequently pronounced in the London region in such a way that it is possible to identify five sound segments: [tl, [s], [h], ['0], [tl. Yet much ofthis phonetic reality may be discarded as irrelevant when it is a question of the structure of the word tot in terms of the sound system of English. Indeed, the speaker himself will probably feel that the utterance tot consists of only three 'sounds', such a judgement on his part being a highly sophisticated one which results from his experience in hearing and speaking English (and not only because of influencefrom the spelling). In other words, the [s] and [h] segments are to be treated as part of the phonological, or linguistic, unit Itf.l The phonetic sequence [tsh] does not, in an initial position in this type of English, consist of three meaningful units; in other languages, on the other hand, such a sequence might well constitute three linguistic units as well as three phonetic segments.