the meaning of underpainting

[kjc0123]

What is the meaning of underpainting in the following sentences?

Countless Grail-related works contain the hidden letter M�whether as watermarks, underpaintings, or compositional allusions. The most blatant M, of course, is emblazoned on the altar at Our Lady of Paris in London, which was designed by a former Grand Master of the Priory of Sion, Jean Cocteau.

[woofgang]

Its putting a painting under the one that you see.

Here's a website

http://howtopaintavermeer.20m.com/underpainting.htm

[Peter Pedant]

If there's picture of you, say,

and some one goes and paints a picture of me on top of it,

then you are the underpainting

[Peter Pedant]

Hi gh Kay jay

and then you're gonna say - but I cant see the picture of me and so what is the use?

But you're the one reading the Da vinci code, kay jay.

And a much better choice if I may say, then some of the real constitutuional c++p you used to take us through