زَيْدٌ and خَالِدٌ are examples of fully declinable proper names; being a proper name is not sufficient by itself.
For عُمَرُ and أَحْمَدُ, check the additional cause specified in the text. Similar spelling alone does not determine diptote status.
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Not all names are diptotes
Restoring Declension: Indefinite Names, Al- and Iḍāfah
زَيْدٌ and خَالِدٌ are examples of fully declinable proper names; being a proper name is not sufficient by itself.
For عُمَرُ and أَحْمَدُ, check the additional cause specified in the text. Similar spelling alone does not determine diptote status.