Strapless Bras are those made without shoulder straps or with easily removable straps. The strapless bra is the most technically challenging to design, because without straps, they must rely completely on the strength of the fabric, the design of the band, and the shape of the underwires to stay in place and provide support.
A strapless bra without underwires is a bandeau and has no significant support. The most successful strapless bras have a wide band; boning under the arms and at the center front; deep underwires; gripper-like silicone strips along the inside to reduce slippage; and special elastic with exposed small rubber loops to help grip. Some strapless bras also put boning in the cups. Such boning gives additional support to larger breasts. Strapless underwires are different from other underwires.
Not only are the wires they taller, but they also curve in to encompass the breast more than a normal underwire. When the underwires splay, they actually encircle the breast giving better support as well as reduced slippage. Even with all this technology and sophisticated designing, a strapless bra will always be a little uncomfortable and need to be pulled up throughout the evening if the wearer is active.