What is muthamasilain?
Answer
Al-mutamaathilaan
or Al-mutamaathilayn is the term uses for two letters which share the same articulation points and all of the same characteristics, meaning, they are both the same letter. The two words, al-mutamaathilaan and al-mutamaathilayn are the same in meaning, it is Arabic grammar and the word's place in the sentence structure which determines whether it is with an alif or a ya'. Please see:
http://www.abouttajweed.com/the-two-same-similar-close-and-far/the-two-the-same-the-two-similar-the-two-close-and-the-two-far-p-3.html
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