Most indefinite nouns take all three cases (triptotes); however, there are also nouns, which do not have the nunation and form only two cases (diptotes) when used نَكِرَةٌ (indefinite). However, they become triptotes when used مُعَرَّفٌ (definite):

In the vocabulary lists diptotes are marked with Ḍamma at the end of the word: خَزَائِنُ. Consequently, indefinite diptotes have no Nunation and they have the same ending in the genitive and accusative. They are mostly broken plurals and adjectives of the form أَفْعَلُ / فَعْلَاءُ, which often refer to colors or bodily defects.

١٢، ١٣، ١٤، ١٥، ١٦، ١۷، ١۸، ٢٠، ۲١

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