underdrawing

noun3

1. •
   A sketch done on the ground of a work of art before paint, pastel, ink, or other more permanent medium is applied.

   • When subjected to printing or photocopying, the blue underdrawing will disappear, leaving only the ink lines. — 2013
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.

   • You can use a croquis as an underdrawing for fashion drawing — 2016
3. •
   An underdrawn cover or lining of the underside of a floor or roof with plasterwork, boarding, or similar.

   • Tongued and grooved boards ⅝ in. thick form the underdrawing for the roof. — 1950