كِتَابٌ وَاحِدٌ
كِتَابَانِ اثْنَانِ
Kitābun already means one book; wāḥidun describes or confirms it.
Kitābāni is already dual; ithnāni reinforces the number.
In these patterns the following noun is not an independent number-tamyīz.
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One and Two Usually Describe the Noun
Number Nouns: Rules from One to One Thousand
كِتَابٌ وَاحِدٌ
كِتَابَانِ اثْنَانِ
Kitābun already means one book; wāḥidun describes or confirms it.
Kitābāni is already dual; ithnāni reinforces the number.
In these patterns the following noun is not an independent number-tamyīz.