Tanwīn is the double ḍammah, kasrah or fatḥah at a word ending. It produces the sound of a sākin nūn in pronunciation, although an independent nūn is not normally written.
The source gives قَدِيرٌ، بِرَسُولٍ، بَصِيرًا for the three double-vowel signs. The audible nūn has to be considered when joining words; its treatment at a stop differs.
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Tanwīn is an audible nūn, not a written nūn
Meeting of Two Sākin Letters — ʿAlā Ḥaddih, Ghayr Ḥaddih and Tanwīn
Tanwīn is the double ḍammah, kasrah or fatḥah at a word ending. It produces the sound of a sākin nūn in pronunciation, although an independent nūn is not normally written.
The source gives قَدِيرٌ، بِرَسُولٍ، بَصِيرًا for the three double-vowel signs. The audible nūn has to be considered when joining words; its treatment at a stop differs.