The book marks ṭūl with rawm and tawassuṭ with rawm as inadmissible combinations in this setting. Rawm retains a trace of the final vowel, so it is not treated like a full stop with sukūn.
Later in this section the author qualifies some uses of 'not permissible' as dispreferred or defective for skilled reciters. Do not erase that terminological nuance.
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Why not lengthen with rawm?
Madd Lengths and Permissible Combinations — Ṭūl, Tawassuṭ, Qaṣr and Consistency
The book marks ṭūl with rawm and tawassuṭ with rawm as inadmissible combinations in this setting. Rawm retains a trace of the final vowel, so it is not treated like a full stop with sukūn.
Later in this section the author qualifies some uses of 'not permissible' as dispreferred or defective for skilled reciters. Do not erase that terminological nuance.