Hi Tom here, in this Circle Line Art School drawing
I'll show you one way to draw a double impossible triangle
Step 1: Draw a small triangle.
Step 2: Extend each of the lines of the triangle.
Step 3: Draw three more longer lines, parallel to the edge of the original triangle.
Step 4: Draw three more lines, from each point of the last lines, parallel to the next side of the triangle, stopping short.
Step 5: Draw three short lines to finish the first impossible triangle.
Step 6: Draw a new, larger triangle.
Step 7: Draw three more short extension lines.
Step 8: Draw three more long parallel lines.
Step 9: Draw three more parallel lines, stopping short.
Step 10: Draw three short end lines.
Step 11: Extend edge line of one of the corners of the smaller impossible triangle.
Step 12: Draw a second short line, parallel to the first.
Step 13: Draw another short parallel line, to the inner line of the large triangle.
Step 14: Draw a connecting line.
Step 15: Repeat these four lines for the other two corners of the original impossible triangle.
Now we can add some simple flat shading, dark on one side mid tonal value on another, and the third side is white.
To learn more visit my website:www.circlelineartschool.com