This Women's PhD Seamless Racerback Bra is perfect to wear while you are going for a run or going to the gym. The inside layer swaps out polyester for merino wool. The wool is soft against your skin and Smartwool was able to transition between the inside and outer layers without any pesky seams under your arms, at the shoulders, etc, which is an amazing feat. What this means to you is comfort and absolutely no chaffing. The dual layer design was not as breathable as single layer sports bras. To keep the sports bra in place, there is a wide elastic band that is covered by the inner and outer layers of the bra. Best for small cup sizes/ low impact activity.