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A bra without straps, relying on an extra-wide band for breast support.
They are designed to be worn with strapless clothing that would otherwise expose the bra straps. They achieve their strength through a longer underwire that encompasses more of the breast, and cups with added padding, boning, and shaping panels. Some have rubberized or silicone beading inside the top edge of the cup to help keep the bra attached to the breast. Some convertible bras allow the straps to be removed, making them into strapless bras.
Don't confuse strapless bras with tube tops or bandeaus. A strapless bra has cups, while tube tops and bandeaus don't.
See also
This tag implicates bra and strapless (learn more).