underdrawing

Short definition from Wiktionary

Noun

1. 1
   A sketch done on the ground of a work of art before paint, pastel, ink, or other more permanent medium is applied.
2. 2
   An image of a figure that is slid under the page on which a designer draws a garment, so that the result conforms to the figure's proportions.
3. 3
   An underdrawn cover or lining of the underside of a floor or roof with plasterwork, boarding, or similar.