Someone by the name of Lola asked for help in figuring out how dart moving works; as such, I’ve tried to answer her specific questions. If someone else has a specific question that isn’t answered here, please let me know. (And Lola, if this doesn’t clear things up for you, let me know!)
It can be very hard to understand dart-moving in the abstract, so I suggest you work the examples as you read.
Start with a square of paper. Draw a dart out from the center. Here I used a dart that was 2″ wide, but it really doesn’t matter.
Cut the dart out.
Close the dart, and tape it shut, as if you had sewn it shut.
See the shape it makes? Now un-tape it and lay it flat again. Make a straight slash, from the tip of the dart to the otherside of the paper.
Rotate the two pieces of paper so that the edges of the “dart” meet. Watch as the slash opens up on the other side. As you close the wedge shape on one side, it opens up a wedge on the other side–a wedge that is the exact same size.
Now, try for yourself. Tape the slash closed, and tape the new dart shut. Is it the same shape as the original darted piece? Yep! Un-tape the dart, and keep working.
Now slash it again, this time from the center point to the right.
Again, rotate the piece of paper to close the dart and open the new slash. The new dart is again the exact same size as the other two darts.
Tape together your old slash, and your new dart. Is it the same shape? Yep!
You can slash it again,
and rotate it again,
but you’ll still always wind up with the same shape when you tape it together. No matter how or where you slash it, you are always removing the same thing, and just moving the other pieces around.
You can join the pieces together, in terms of pattern drafting, in different ways. You can butt the pieces together, and cut them as one from a piece of cloth. Or you can seam them together. But it’s always the same amount of fabric, with the same amount taken away, and it will always make the same shape.
So what are some practical examples?
I made a card-board cut out of a basic bodice sloper, and traced around it as I manipulated the darts. (In some places it appears the darts don’t meet; they do, but my pen wasn’t narrow enough to get in the cracks when tracing.) You probably have quarter-scale samples of slopers or blocks in your drafting textbook, and you can trace them out and work along with these sameples to better understand how it works. Here’s some basic bodices:
I don’t know how your drafting method had you draft your basic bodice. Some have just the horizontal dart, some have both darts, and some have just the vertical dart. Any one of them can be changed into the other. In the picture above, I show how to take a bodice with only a vertical dart and change it into a bodice with only a horizontal dart: slash a line for where the new dart will be. Rotate that corner of the pattern. The middle image shows what the pattern looks like while rotating the piece; if you stop right there, you will have a bodice with both a veritical and horizontal dart. If you keep rotating it, you will wind up with just the horizontal dart.
Just like with our practice square, it still has the same amount that is there, and the same amount that is not. It’s just re-arranging it. If you sewed together the bodice on the left, it would fit exactly like the bodice on the right—or the bodice in the middle.
How about when a garment doesn’t have darts—only seams? Where is the shaping then, and how can it fit like a garment with a dart? Look at this sample:
Here we start with a bodice with one vertical dart. We slash open to the shoulder line. We rotate the pattern piece half-way, so we have one vertical dart from the top pointing down and one vertical dart from the bottom pointing up. Two darts, the same amout of shaping as in the first image. But what happens when we simply seperate the two halves of the pattern?
Princess seams! There will be no “darts” in the garment, just “seams”. But the same amount has still been taken away, and the fit is still the same. This often referred to as “darts hidden in the seams” or “shaping in the seams”. When the garment is sewn up, you won’t see a “shaped” seam; you’ll see a “straight” seam. But if you take the peice apart, and lay them flat, you will see that the pieces don’t match; there’s space between them–darts!
Darts can moved to just about any place imaginable. Here a few examples; all of them are perfectly valid and will produce the same fit.
This sequence starts with one horizontal dart, in the typical under-arm posistion. It can be rotated to the center front. Or it can be rotated to the neckline. All three bodices will have the same fit; the same amount is always taken away, and the rest is just rearranged.
You don’t have to have only one dart. You can have two darts,
or three darts,
or, if you want, even more. When the darts get small enough (because there is so many of them; the more darts you make the smaller each dart will be, because you are never taking away more—just redistributing it), they are referred to as tucks.
Now, at this point, you may have stumbled upon a problem. When we practiced with a square, it was easy to see that every new wedge opened up was the same size as the old wedge that was closed. But when you rotate darts on an actual bodice, it looks like the darts wind up all different sizes–smaller darts horizontally and larger darts vertically.
Let’s look at another paper example. Here’s another piece of paper with the same two inch dart taken out. But this piece of paper isn’t a perfect square; its a rectangle (legal sized paper, to be exact!)
If we tape this dart together, it look simlar to the shape we made with the square piece of paper. The only difference is that it has a lot more length on one side.
Un-tape the shape, and slash and rotate as before.
Looks scary, doesn’t it? The second dart looks so much bigger than the first dart, it’s hard to imagine it can possible get taped into the same shape again. But watch this:
The wedge still fits! All the length past the wedge is length that is longer than the original square, but the new dart is still the same angle. If the paper was equally long on either side of the dart point, both darts would be the exact same size in angle and length and width.
And, if you tape the slash and the new dart, you will see that you still have the same shape you started with.
Now, the question you asked about was actually concerning skirts, so let’s look at some skirt examples.
The image on the left is a basic front skirt sloper with one dart.
The middle image shows that if you made a slash straight down from the dart point and seperated the pattern pieces, you could have a princess seamed skirt. You wouldn’t “see” a dart in this skirt if you sewed it up (just a straight seam), but it would still have the same shaping. By adding a superflous straight seam at the end of the dart, it disguises the dart as just being part of the seam.
The image on the right shows what happens when you close the waist dart and open up a dart to the hem. Just like on our rectangle example, the dart appears much larger, but it’s really the same angle. If you sewed that dart up, the last image would fit just like the first image. But if you leave that dart un-sewn, you would be adding fit near the waist (subtracting fabric away from there), and fullness (adding more fabric) to the rest of the skirt. In otherwords, you would be making an A-line skirt–fitted through the hips, and then sweeping out!
I’m not 100% sure what you were referring to by “a bell-shaped skirt”. Perhaps you just meant an A-line skirt. Or you might have meant a style like this:
My drawing skills aren’t so hot, but you can see a rough sketch in the lower right corner what this pattern would sew up like: It would be fitted (and curved) through the hips, fall straight for your desired distance, and then flare out into almost a flounce. The dart is still there; a line is slashed straight down, and the pieces are seperated. Then equal wedges of fabric are added near the hemline on every seamline. The seam lines are all the same length as their corresponding pieces. When you sew them together, they won’t look like shaped seams–they’ll look like straight lines, as in my sketch. It’s only with the piece laid flat that you can see there is fabric subtracted near the waist, and fabric added near the hem.
Does that help?
Oh, and as a final note, when someone speaks of “rotating the dart to the side seam”, they are essentially doing this:
I cut our original triangle in half, to show how it’s still the same amount being taken out. The one difference, though, is that darts point to the bulge. Since in this example the extra fabric is being taken out at the sides, instead of the middle of the fabric, you may find the fit changes.