Over the weekend, the American fashion designer shared never-before-seen photos of the custom design, revealing every stunning detail.
The back was designed with buttons trailing down the spine with the silky fabric flowing out from behind in the shape of a cape.
Just like her first dress of the day, the gown was created with a low back to avoid covering Princess Eugenie's scar.
Posen captioned the photo, "The back by popular demand, before the final #whiteroseofyork embroidery was placed."
Needless to say, Zac Posen can design our reception dress any day.