new languages, such as pidgins and creoles. Imperfect learning: According to one view, children regularly learn the adult forms imperfectly, and the changed forms then turn into a new standard. Alternatively, imperfect learning occurs regularly in one part of society, such as an immigrant group, where the minority language forms a substratum, and the changed forms can ultimately influence majority usage. Social prestige: Language may not only change towards features that have more social prestige, but also away from ones with negative prestige, as in the case of the loss of rhoticity in the British Received Pronunciation accent. Such movements can go back and forth. According to Guy Deutscher, the tricky question is "Why are changes not brought up short and stopped in their tracks? At first sight, there seem to be all the reasons in the world why society should never let the changes through." He sees the reason for tolerating change in the fact that we already are used to "synchronic variation", to the extent that we are hardly aware of it. For example, when we hear the word "wicked", we automatically interpret it as either "evil" or "wonderful", depending on whether it is uttered by an elderly lady or a teenager. Deutscher speculates that "[i]n a hundred years' time, when the original meaning of 'wicked' has all but been forgotten, people may wonder how it was ever possible for a word meaning 'evil' to change its sense to 'wonderful' so quickly." Types Phonetic and phonological changes Sound change—i.e., change in the pronunciation of phonemes—can lead to phonological change (i.e., change in the relationships between phonemes within the structure of a language). For instance, if the pronunciation of one phoneme changes to become identical to that of another phoneme, the two original phonemes can merge into a single phoneme, reducing the total number of phonemes the language contains. Determining the exact course of sound change in historical languages can pose difficulties, inasmuch as the technology of sound recording dates only from the 19th century, and thus sound changes before that time must be inferred from written texts. The orthographical practices of historical writers provide the main (indirect) evidence of how language sounds have changed over the centuries. Poetic devices such as rhyme and rhythm can also provide clues to earlier phonetic and phonological patterns. A principal axiom of historical linguistics, established by the linguists of the Neogrammarian school