وَنُونُ التَّأْكِيدِ الْخَفِيفَةُ لَا تَدْخُلُ فِي التَّثْنِيَةِ أَصْلًا.
Translation: The light emphatic nūn does not enter the dual.
The book excludes light nūn from the dual because a silent nūn after alif creates an impermissible meeting of two sukūns; moving it removes its defining lightness.
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Light Nūn Is Excluded from the Dual
The Two Emphatic Nūns: Forms and Rules
وَنُونُ التَّأْكِيدِ الْخَفِيفَةُ لَا تَدْخُلُ فِي التَّثْنِيَةِ أَصْلًا.
Translation: The light emphatic nūn does not enter the dual.
The book excludes light nūn from the dual because a silent nūn after alif creates an impermissible meeting of two sukūns; moving it removes its defining lightness.