How to Draw Bottles that are Symmetrical

After struggling with a crooked bottle in a still life painting with lopsided shoulders, this morning I figured out how to draw bottles and keep the curves and angles on both sides symmetrical. 

The solution: 

1. Make a mark for top and bottom and widest point of width on each side and draw a rectangle enclosing the shape. Draw a vertical line down the center of the rectangle.
2. Next I  lightly drew a rectangle (or enclosing envelope) around each “section” of the bottle, with the bottom of each rectangle at the spot where the exterior of the bottle changes direction from a curve in or out. I left some of those marks in the sketch above.
3. Then draw straight diagonal lines from section to section and soften them into matching curves. It’s much easier to draw straight lines accurately than curved ones.
4. Draw the ellipses for each section using the guidance here from Sadie Valerie.