I was commissioned to make 2 helical hand rails for a
brown stone home renovation in Boston, Mass, in mahogany
wood. A sample was provided so I could make a template
of the flat surface on the underside of the rail.
Below is my base surface template. I then had to
make the rails in two pieces and make each piece match
the template for my first reference surface.  
The two pieces are shown here and you can see one of the patterns
I used to carve the rails to the required shape. You can barely make
out the many pencil lines on the rail where it matched the patterns.
This was so I would know where it had matched the pattern and
where I had to continue carving. 
You can see the compound curves in the rail pieces that are
always referenced from the flat bottom surface. The erratic grain
of mahogany with the many curves made this carving part difficult.
The rail pieces were connected with railing bolts and glue and then
sanded from a 60 grit up to a 320 grit. I had to make up 5 shaped
sanding blocks to match the required contours of the shape and 
also maintain consistancy of the shape. 
This picture shows the "U" shape of the rail as it rounds the corner
from the rise to the landing. It's amazing that each piece, with it's
compound curves, will fit within a 4 1/2 thick piece of wood.