Mona Lisa replicas and reinterpretations
After restoration, the Prado's "Mona Lisa" revealed details covered by previous restorations and layers of varnish.
Furnishings and fabrics were enhanced, as well as landscape and facial features.
It is anticipated that such revelations may offer further insight into Leonardo's original.
Experts at the Louvre reportedly supported the Prado museum's findings.
The Prado replica was subsequently transported to the Louvre in 2012 to be displayed next to "Mona Lisa" as part of a temporary exhibition.
Two nude paintings bearing similarities to Leonardo da Vinci's original were part of a 2009 exhibition of artwork inspired by "Mona Lisa".
Displayed at the Museo Ideale in Leonardo's hometown of Vinci, near Florence, some believe one of the paintings – dating from Leonardo's time – to be the work of Leonardo himself, and it has at times been credited to him.
Other experts theorize the painting, one of at least six known to exist, may be just another copy painted by "followers" of Leonardo.
Scholarly dispute persists as to artist, subject and origin.
The nude in question, discovered behind a wall in a private library, reportedly belonged to an uncle of Napoleon Bonaparte, who owned another of Leonardo's paintings.
Facial features bear only vague resemblance, but landscape, compositional and technical details correspond to those of the "Mona Lisa" known worldwide today.
Portrait of Dorothea Berck
Portrait of Dorothea Berck is a 1644 painting by Frans Hals that is in the collection of the Baltimore Museum of Art. It depicts Dorothea Berck at age 51, the wife of the prosperous Haarlem merchant Joseph Coymans, whose portrait Hals also painted. Both paintings were executed on the occasion of their daughter Isabella's wedding, whose marriage pendants Hals also painted.
The painting, like many of Hals's portraits, is inscribed and signed, but there is no mention of the identity of the sitter, and it was only identified as Berck in 1908. 
The portrait is noted for its classical similarity to the Mona Lisa. Like the Mona Lisa, she sits upright in a dark chair and rests her hands on either a dark desk or perhaps a large dress that surrounds her. Her ungloved hand may be clasping a book or part of her sleeve. The loose brushwork is typical of Hals and shows a fleeting gesture with the casual way her finger is hooked over her wrist. Portraits of subjects with one glove on and one glove off are common in Dutch 17th-century marriage pendant portraits. Gloves symbolize the wedding vows where the husband and wife each hold one of a pair of gloves. In this case, her husband Joseph Coymans is wearing the "other glove" in his pendant portrait. In Hals' marriage pendant portraits, the husband is always situated on the left and the woman on the right, and the light always comes from the left, shining fully on the woman.