Modern high-tech fibers have made it possible to fabricate bras from completely seamless lightweight materials that can be molded and shaped for a variety of bra styles. This is especially important in t-shirt bras, meant to basically disappear under tees and other knit fabrics.
Smoothing t-shirt bras will have seamless cups, usually molded and possibly with some padding. It may also have a hidden underwire, seamless sides and a smooth stretch back or leotard back. Smoothing t-shirt bras are available in a huge range of sizes and styles, from minimizers to plunge to push-up.