Underwire Bras have a firm plastic or metal underwire that runs around the bottom of the breast cup, giving shape and structure to the bra. Perfect for all styles and breast types. - An underwire bra has a metal or firm plastic underwire which runs along the bottom of the cups, enclosing your breast. - An underwire should never ever touch your breast tissue, but should only touch your ribcage. - An underwire gives support and structure to your bra, and gives the fabric a firmer base to support your breasts.