This change in pronunciation results from coarticulation. In speech, we anticipate a sound while producing the preceding sound. In this case, while producing /n/ we begin to form /b/ for which lips come together, resulting in /m/. There are other such examples where /n/ becomes /m/ when preceding a bilabial stop: • penbe à pembe • anbar à amba • saklanbaç à saklambaç • cünbüş à cümbüş As seen in these examples, this process is reflected in the orthography. ‘İstanbul’ is an exception as it is written with the letter ‘n’ but this could be because ‘İstanbul’ is a proper noun.