Dual Forms of the Three Classes

Derived Nouns: Noun of Instrument (Ism Aalah)

Each singular form has a corresponding dual form.

DualSingularClass
مِفْعَلَانِمِفْعَلٌSughraa
مِفْعَلَتَانِمِفْعَلَةٌWustaa
مِفْعَالَانِمِفْعَالٌKubraa

So the class stays the same while the noun changes from one instrument to two.