The Evenly Subtle Shaper Insert (BB0) is ONE silicone bra filler that mimics the weight of natural breast tissue so that your bra can distribute the weight of your breasts and insert evenly in the cups. BB0 works best for breasts with difference of 1 cup size or less; great for lumpectomy or for very slight size or shape difference between breasts. This silicone insert is reusable and waterproof, so you can even wear it to swim! Insert can be worn inside pocketed bras, or inside a regular bra directly against the skin if you have a size difference and want the smaller breast to more evenly fill and fit your bra cups to look like the fuller breast! To find the perfect fit, first, you need a well-fitting bra that should fit your bigger breast best. Then, you select the Evenly Subtle Shaper size that matches the difference on your smaller side. A good rule of thumb to determine what THICKNESS you need: 1 cup size = a bra gape of approximately 0.5cm. Once you know which thickness you need, check the Evenly sizing chart to select the number size (based on your UK bra size). If you have a larger difference of more than 1 cup then wed recommend the Bra Balancer instead of Subtle Shaper.