There are a few different types of bras that can be worn with an open-back sweater. A strapless bra is one option, as it will not show any straps when the sweater is worn. Another option is to wear a regular bra and simply tuck the straps in so they are not visible. A racerback bra can also work well with an open-back sweater, as the straps will not be visible from the front. Whichever type of bra you choose, make sure it fits well and provides adequate support.