The impact of a picture (or group of pictures) is enormously affected by how it is framed. Very few pictures in the Gallery are still in their original frames, although with altarpieces it is often possible to make an educated guess about the kind of complex framing structure they once had.
Given that a good deal may be known about the original framing, the National Gallery tries to give pictures appropriate period frames where possible. In some cases frames are specially bought, or replicas are made.
However, when a painting is in an important frame given to it at a later date this has often been retained as part of the history of the picture. An example of this is Poussin's 'Adoration of the Golden Calf' which has one of the most sumptuous and exquisitely detailed early 18th-century French frames known - although Poussin is known to have favoured simple frames.