Vowel harmony
Vowel harmony is the principle by which a native Chuvash word generally incorporates either exclusively back or hard vowels (а, ӑ, у, ы) and exclusively front or soft vowels (е, ӗ, ӳ, и). As such, a notation for a Chuvash suffix such as -тен means either -тан or -тен, whichever promotes vowel harmony; a notation such as -тпӗр means either -тпӑр, -тпӗр, again with vowel harmony constituting the deciding factor.