We are very excited about these foam cups! It has taken us a lot of study to understand what should be in a foam cup, and these fit the bill. First, they are an excellent quality and made right here in North America. Second, they work perfectly with the 2 T-shirt bra patterns that we carry, both the Anita Amelia and the T-Shirt bra pattern from Bra Makers Supply. This means that you're buying more that a cup, you're buying the ability to make a bra. They are sided, which means that there is a left side and a right side rather than just popping them in and out. Finally, the sizing of the cup roughly corresponds with our wire sizing so you know where in the size range to target. There might still be some need to get a different size, but at least you know the neighborhood ;). T-shirt bras require a knit fabric for the cups, and our Microlycra is an excellent choice. We'll be creating T-shirt bra kits over the coming months, but in the meantime you can ask me to substitute the Microduoplex for Microlycra in any kit in the notes and I'm happy to do so.