The first three classes are called umm al-abwāb / principal classes in this text. Their middle-radical vowels change between the perfect and imperfect:
ClassPerfect middle vowelImperfect middle vowel
Naṣaraau
Ḍarabaai
Samiʿaia
These classes are also very common. 'Principal' is a technical classification, not a claim that the remaining three are defective or invalid.
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Properties of the Six Basic Triliteral Forms: Contest, Transitivity, and Semantic Patterns
The first three classes are called umm al-abwāb / principal classes in this text. Their middle-radical vowels change between the perfect and imperfect:
ClassPerfect middle vowelImperfect middle vowel
Naṣaraau
Ḍarabaai
Samiʿaia
These classes are also very common. 'Principal' is a technical classification, not a claim that the remaining three are defective or invalid.