Tanwīn is absent with the definite article, on the first member of iḍāfa, and on diptotes. These are distinct reasons; only the last is inherent diptote status.
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Common Reasons for No Tanwīn
Tanwīn: Five Types, Their Function, and Omission
Tanwīn is absent with the definite article, on the first member of iḍāfa, and on diptotes. These are distinct reasons; only the last is inherent diptote status.