Continuing with the "Fitting Dresses" booklet from the U.S. Department of Agriculture.
Darts can be useful to take up fullness and to shape and direct fullness where needed. Tucks and gathers do the same with softer effect. When you use darts, try different places until you get a smooth, becoming line. In fitting, rarely take up as much as 1 1/2" in fullness in one dart. Two or more smaller darts are usually better than one large dart. the smaller the amount of material to be taken up, the shorter the dart can be.
1. Front shoulder darts remove wrinkles from the shoulder down to the bust. They also straighten the grain of goods. Direct the shoulder fullness or darts toward or over the bust.
2. Back shoulder darts from the back shoulder seam to the shoulder blade give ease across the back. They improve the fit on a round-shouldered person.
3. Neck-line darts, tucks, or gathers at the back make a dress fit snugly at the neck and yet allow fullness over the back. Such darts should be straight or diagonal. They may be stitched on the right or wrong side.
4. Underarm darts give ease and can be used to shift the crosswise grain of goods below the bust. Usually one or more darts from the underarm seam are placed below but pointing to the bust line. Generally, any one underarm dart should be not more than 3/8" wide or more than 3" long.
5. Waistline darts are often placed in a blouse at points directly below the bust or shoulder blade to give needed ease.
6. Skirt darts from the waistline in the side back or front make for smoothness between the hip and a snug waistline. Do not space back skirt darts too far apart on a figure with broad hips.
7. Elbow darts properly located give elbow room and hold fullness in the right place. When locating such darts, bend the arm to find the elbow point and place the darts or gathers at the bend.
For the most part, darts are darts. They help give fullness when fullness is needed. What do you think of the advice not to have a dart take up more than 1 1/2"? What do you think of the advice of having 2 or more darts when the take up is 1 1/2" or more? What do you think of the advice of the underarm dart not being more than 3" long? I would imagine that this advice doesn't apply to French Darts.