From Knitty:

This DK-weight tank top serves double-duty as a nursing bra. A hook-and-eye closure at the top of the keyhole at the front allows the neckline of this top to sit higher than other nursing tanks. Undo this hook plus the closure at the top of the drop-down cup to easily feed your little one through triangular nursing openings hidden inside the cup.

This top features separate sizing for the body and for the bra cups, allowing you to put a D cup on a small body if necessary (isn't nursing great?)