It looks like tiffany put in the least effort to maintain her armpits even though she isn't shy from showing them off more than often. Imperfection is a perfection. Taeyeon's are the opposite. Her armpits are unbelievably smooth; you gotta see it to believe it. Yoona's armpits are pretty average, really. However she is so thin that you could see the bones at her shoulder joints whenever she raised her arms. It just makes you want to protect her and her armpits.