Once I remember reading something about how part of the distinctive sound of country-western music comes from its vocal harmonies. I suspect that the harmonies are part of why people are so divided about country music, love-it-or-hate-it.
I even saw an electronic keyboard with a bunch of harmony presets, and one of the options was “Country”. So what are these harmonies? I mean, harmonizing in thirds isn’t particularly unusual … do country dueters harmonize in fifths and sixths or something? Or do they sing sharper or flatter than other people would?
Country musicians in duets typically harmonize in thirds or sixths but that is relatively constant among pop/country/rock/folk musicians everywhere. Country musicians typically play in 2 or 3 time, have a thick accent (for the most part) that is discernable through singing, and often use fiddling style if they have a violin (which many of them do). It is not really a question of harmony but rhythmic and melodic style.
Vocally, in most discernibly Country singing, there’s always a slight vestige of the yodel that’s somewhere in it ancestry. That little flip between a high note and a low note, that in Alannis Morisette sounds like the bark of a clubbed sealpup. You hear a lot of it in Hank Williams, Patsy Cline, and especially Buck Owens. I think that’s partially what is referred to as “twang.”
As a sometime singer, I can tell you that it happens when you transition abruptly, without a slide, from chest voice to falsetto; perhaps someone else can supply some physiological terminology for this phenomenon.