Pólya's Space-Filling Curve
Generating Pólya's space-filling curve begins with a right triangle. For each iteration, split each right triangle into two smaller right triangles and join the centers of the resulting smaller triangles.
Snapshots 1, 2: adjusting the opacity of the triangles and the curve highlights the construction
Snapshot 3: for an isosceles right triangle, the Pólya space-filling curve is identical to the Sierpinski curve
H. Sagan, Space-Filling Curves, New York: Springer-Verlag, 1994 pp. 63–67.