A woman's sack back and petticoat of ivory silk tobine with blue vertical decoration, brocaded floral sprays in shades of pink, green and red. The sack back is open at the front with elbow-length sleeves. The bodice and sleeves are lined with linen. The bodice meets at centre front. At the back are two double box pleats stitched at the neck line. The sack back is made of 3 widths of silk with 2 partial panels and 2 triangular gores on each side of the front. The skirts are shaped for a French, fan-shaped hoop. A waistseam runs from the front edge to the bodice side-back seam. The skirts are pleated on either side of the pocket openings in the side seams.
The petticoat is made of 6 widths of silk, with a box pleat at centre front and flat pleats at the side and back. The waist is bound with linen tape, which runs through a casing formed of a deep hem of silk on either side of the centre back opening, and forms the fastenings. The hem is faced with white silk taffeta.
The sack back and petticoat were probably made in the 1770s. In the early 1780s, the style was updated. Sleeve ruffles and ruchings on the sack back and petticoat were removed. The robings were unpicked and the bodice fronts reconfigured. The facing of the sack back hem was removed.