The princess line is defined by a bodice and a skirt which doesn't have a waist seam.
The formal frock coat only buttons down to the waist seam, which is decorated at the back with a pair of buttons.
And that applies also to the 1940's-style dresses with bust darts and waist seams after last year's slip dress.
Since around the 1830s the coat has been constructed with a waist seam that allows greater waist suppression.
During the Regency, the fashion was to have very form-fitting clothes, with sidebodies, waist seams, and a flared skirt.
Frock coats remained fashionable, but new shorter versions arose, distinguished from the sack coat by a waist seam.
Kirtles began as loose garments without a waist seam, changing to tightly fitted supportive garments in the 14th century.
Later kirtles could be constructed by combining a fitted bodice with a skirt gathered or pleated into the waist seam.
Italian sleeveless dress of mid-15th century has an obvious waist seam and a skirt pleated to the bodice.