The recognition of the systematic nature of the interference of the mother tongue when learning a second language is among the most significant advances in linguistics for the teaching and learning of foreign languages. The work of Weinreich showed that interference between language systems--the absorption of loan words, calques, and phonological, morphological, and syntactical features from one language to another--did not happen by some mysterious process in mid-air or in mid-history, but in the nervous systems of bilingual speakers. The more individuals there were who were bilingual in the two languages of the region, and the greater their community prestige, the more chance there was of their own "interfered-with" speech habits being adopted by monolingual speakers of the surviving language. Rather than linguistically analyzing and comparing two languages to predict possible errors on the part of the learner, the author feels it safer to analyze the errors that are actually made and then start looking for the reasons in the system. He warns that "systematic perceptual distortion" in both pupil and teacher can produce faulty diagnosis of an error where an awareness of structural factors is lacking. An illustration of misdiagnosis on the phonological level is given in a contrasting of English and Urdu stop consonants. (AMM)