Shapes - mine and not-mine

A picture is worth 1000 words. Therefore, behold:

Marked up ineptly in Skitch

The yellow outlines are the cup pattern pieces for Harriet. The pink outlines my TNT cup pattern. There are two things that occur to me.

• The shape at the bottom is very different. 
• The "dart" at the top is very different.

Bottom Edge

The lower edge of the Harriet cup is relatively flat (black arrow) whereas my TNT pattern has quite a curve (red arrow). Harriet curves up, but only a bit, at the seam between the lower cup and the upper cup. My TNT pattern curves deeply and smoothly throughout the entire bottom line of the cup.

Dart

When I say "dart", I mean the space between the top edge of the lower cup piece and the bottom edge of the top cup piece.

To assist, I marked the ends of the dart on the Harriet pattern with orange exclamation markers (the marker should actually point at the little dot on the upper edge of the lower cup) and those on the TNT pattern with green check mark markers.

Imagine the two marked shapes sewn together in three dimensions. We have a lightly darted cup with a lot of curve in the lower edge of the cup (TNT) and a deeply darted cup with a relatively flat lower edge (Harriet).

What is the effect of these differences?

Harriet cup projection - side view

When the lower edge of the Harriet cup is sewn to the frame of the bra, the lower cup projects outward. Because the lower edge is so flat, it's like sewing a cylinder into a flat piece with a circular opening. If you think of the frame as a flat plane, the cup pieces would emerge from it almost at a 90 degree angle. This can be seen at left.

Harriet cup projection - view from the bottom of the band

The big dart in the Harriet cup is formed at the top edge of the lower cup (notice that the seam between the two lower cup pieces is quite flat). The apex of the dart is more or less where the two cup seams converge. Have a look at the photo to the right.

By contrast, when the already-curved edge of the TNT cup is sewn into the same frame shape, it is not forced upward at a right angle, but rather forms a slope. The highest point of the cup is lower (less of a dart) and the continuous curve of the dart means that the shape of the cup is less pointy.

I prefer to get my roundness from the support of a flatter lower cup sewn in stretch fabric than from the pre-defined projection of a cup sewn in fabric with low or no stretch. Lucky for me, I'm small enough that the former works.

Therefore, my next trick is either going to be a mash-up or something completely different, to cleanse my sewing palate.