One of the main element of a painting is the light source. In the Mona Lisa, there poses a dilemma: the shadows cast from her nose do not correspond with the background lighting. It seems that the background and the sitting area are independent in lighting with each other. It is explained by Leonardo’s painting habit. He sketched and noted parts of his subject separately (see illustration below) and later assembled them in his studio in order to preserve the secret of his Sfumato technique.

(The Notebooks of Leonardo Da Vinci, Jean Paul Richter, 290)

The background is transformable because it changes from one Mona to the other to suit the subject. We will discuss more about London’s Mona background later on. Compounding to the lighting source problem above, it is impossible to find the painting’s varnishing point(s) or a clear horizon because of the lack of indicative lines while we try to back track its perspective composition.

Let's start from the beginning and clear a few misconceptions:
The Louvre's Mona Lisa was originally painted on wood panel and later transferred onto canvas by Louvre's experts. The construction of the panel and how to prime it was fully illustrated and described in his notebook (page 319).