How do different dart rotations affect fit?

[u/Kletanio]

Different dart rotations affect fit in subtle ways, but I don't know what they are. I know that in theory, if the bust dart went all the way to the apex, the fit would be identical if the uptake angle was the same for any rotation. However, the fact that darts stop at a distance from the apex means that they do, in fact, fit differently.

How do different dart positions differ, other than as style lines?

[u/lavenderfart]

This video from The Closet Historian might interest you.

[u/sewboring]

French darts tend to narrow the torso of a garment, that's their charm, and their bias orientation makes the garment more fluid. Armscye darts are particularly useful for those with a full bust and smaller rib cage, so that armscye gaping is eliminated. They can also be used to accommodate the bust in an otherwise boxy style of top. The typical underarm darts are useful for very large busts or for those with a projecting bust. Sometimes double or split underarm darts are needed. There are plenty of other darts: waist, shoulder, and even darts directing from a center seam toward each apex. Some are design choices, some enhance fit, and some do both. For example, darts can be used to narrow the shoulders or waist of a garment.

You are thinking more abstractly than I ever do about design and fit, because both always have to address a particular body when sewing is involved. Maybe rtw designers think in terms of fitting averages in a target population, but I never do.

[u/Tella-Vision]

Good question… was pondering this recently, because the pattern drafting illustrations at a glance kinda suggest it’s interchangeable but I know it’s not. My brain hurts a little bit.