When there is Nun saakin نْ (including Tanween) and the next letter is Ba ب then the sound of Nun saakin نْ will become silent and substitute with sound of Meem saakinمْ with Ghunnah (nasal sound). Note this rule applies both within a single letter or two words side The most manuscript of the Holy Qur’aan, a tiny Meem ۘ is placed between the letters hinting such sound alteration.

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