- Flip the sewn background triangle right side up. Hold the foundation to the light, printed side facing you. Look for the shadow of piece 2. It should extend far enough beyond the line separating it from piece 3 to create a stable seam when that line is sewn. It should also extend past the outer printed edges of the foundation.
If the position is good, trim excess seam allowance and finger press firmly in place (or press with an iron).
- Align a length of your dark strip right side down against the combined edges of pieces 2 and 3 (illustration, upper right). Hold in place and flip the foundation over. Sew on the long line that separates pieces 1 and 2 from piece 3, beginning and ending a few stitches on either side of the line. Flip upright, check placement, trim excess allowance and press.
- Trim excess strip that extends past foundation (illustration, lower left).
- Place a background triangle against the unsewn edge of piece 3 as shown (illustration, lower center). Hold the triangle in place and turn the foundation over. Sew on the line that separates piece 3 from piece 4, beginning and ending a few stitches on either side.
- Flip upright and check position -- it must extend past the outermost foundation lines for its area. Trim excess seam allowance and press the entire unit.
- From the printed side of the foundation, rotary or scissors-cut through all fabric on the outermost lines (illustration, lower right, shows unit before trimming on lines).
- Assemble one more identical unit on another foundation. Sew the two units together along their diagonals. Remove (temporary) foundations from the diagonal seam, but leave others in place.
- Make a total of sixteen Whirlwind quilt blocks.