from the National Gallery website:
This is a seventeenth-century Dutch ripple frame, crafted from ebony veneer. The back moulding of the box frame is adorned with a ripple moulding, followed by a scotia and a narrower ripple moulding. It has a bold zigzag ripple on the frieze, ending with a ripple moulding at the sight edge.
In 2002 a large box frame was used to make smaller frames for two paintings by Vermeer, A Lady Standing and Lady Seated at a Virginal. Both original and replica metal corner coverings feature on the outside of the frames.