فَإِنْ تَضَمَّنَ الثَّانِي حَرْفًا، يَجِبُ بِنَاؤُهُمَا عَلَى الْفَتْحِ، كَـ«أَحَدَ عَشَرَ» إِلَى «تِسْعَةَ عَشَرَ»
Translation: If the second component contains the sense of a particle, both elements must be built on fatḥ, as in the numbers 11 through 19.
The dual pronoun in بِنَاؤُهُمَا refers to both parts. It is not just the second word whose final form stays fixed. The author immediately gives a special exception for twelve.
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Al-Murakkabāt — Compound Numerals, the Twelve Exception and Declining Baalbek
فَإِنْ تَضَمَّنَ الثَّانِي حَرْفًا، يَجِبُ بِنَاؤُهُمَا عَلَى الْفَتْحِ، كَـ«أَحَدَ عَشَرَ» إِلَى «تِسْعَةَ عَشَرَ»
Translation: If the second component contains the sense of a particle, both elements must be built on fatḥ, as in the numbers 11 through 19.
The dual pronoun in بِنَاؤُهُمَا refers to both parts. It is not just the second word whose final form stays fixed. The author immediately gives a special exception for twelve.