अयोगवाहौ (Ayogavāhau) are Varnas in Sanskrit which always come after vowels. They cannot be pronounced by themselves. There are two such letters- विसर्गः (Visargaḥ) (ः) and अनुस्वारः (Anusvāraḥ) (ं).

विसर्गः (Visargaḥ)
This अयोगवाहः (Ayogavāhaḥ) represents a repetitive sound which has the sound of the letter ह् (H) and the vowel that came before it. Its symbol is this- ः

अनुस्वारः (Anusvāraḥ)
This अयोगवाहः (Ayogavāhaḥ) represents a nasal sound. When it is used for वर्गीय-व्यञ्जनानि (Vargīya-Vyañjanāni), its sound becomes that of the अनुनासिक-व्यञ्जनम् (anunāsika-vyañjanam) of that वर्गः (Vargaḥ). The अनुस्वारः (Anusvāraḥ) can be replaced with the अनुनासिक-व्यञ्जनम् (anunāsika-vyañjanam) of that वर्गः (Vargaḥ).

When it is used for अवर्गीय-व्यञ्जनानि (Avargīya-Vyañjanāni), its sound becomes a pure nasal. In this case, the अनुस्वारः (Anusvāraḥ) cannot be swapped with any other letter.