LessonsAl-Murakkabāt — Compound Numerals, the Twelve Exception and Declining BaalbekQuiz

وَإِنْ لَمْ يَتَضَمَّنْ ذَلِكَ، فَفِيهَا لُغَاتٌ، أَفْصَحُهَا: بِنَاءُ الْأَوَّلِ عَلَى الْفَتْحِ، وَإِعْرَابُ الثَّانِي إِعْرَابَ غَيْرِ الْمُنْصَرِفِ، كَـ«بَعْلَبَكّ»
Translation: If the second part does not have that feature, more than one form of usage exists; the most eloquent makes the first element indeclinable with fatḥ and declines the second like a diptote, as in Baalbek.

Important difference: Both elements are not fixed on fatḥ. The first is fixed, while the second declines like a diptote.

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Baalbek — Mixed inflection

Al-Murakkabāt — Compound Numerals, the Twelve Exception and Declining Baalbek

وَإِنْ لَمْ يَتَضَمَّنْ ذَلِكَ، فَفِيهَا لُغَاتٌ، أَفْصَحُهَا: بِنَاءُ الْأَوَّلِ عَلَى الْفَتْحِ، وَإِعْرَابُ الثَّانِي إِعْرَابَ غَيْرِ الْمُنْصَرِفِ، كَـ«بَعْلَبَكّ»
Translation: If the second part does not have that feature, more than one form of usage exists; the most eloquent makes the first element indeclinable with fatḥ and declines the second like a diptote, as in Baalbek.

Important difference: Both elements are not fixed on fatḥ. The first is fixed, while the second declines like a diptote.