|"Delphos" dress, Mariano Fortuny, 1910-1930; CHM 1997.68.1 (pattern available at link)|
|Mrs. Condé Nast, from Woman as Decoration, Emily Burbank|
The Fortuny heat-set pleating method created unbroken lines of creases down the length of the dress, apparently before the reinforcements under the arms were done. Rows of gathering stitches hold the pleats in place there, where they would be under a lot of stress, and a length of cotton tape is sewn down under the "armscye", giving the dress a little more hidden stability.
As far as I know, there is no chronology of Delphos gowns available. There must be certain techniques or styles or materials that were only used during certain periods of time, but there are so few securely dated gowns or photographs of them that I'm reluctant to draw any conclusions. As a result, my date range is far wider than I would prefer it to be.