The plain foundation slip is really an elongation of the built-up waist lining, with pleats added at the sides over the hips for ease. When putting together your 1920s wardrobe, keep in mind that the plain foundation slip takes just under 3 yards for the average figure! Choose a firm but smooth fabric. If the dress fabric is sheer, a slip of the dress material is preferable.
The following image illustrates a typical 1920s plain foundation slip. This kind of slip can be made with a camisole top if the dress in which it is to be used requires it.
The upper part of the slip is fitted in the same manner as the built-up waist lining, with fitting in any excess waistline fullness by deepening the side seams. French seams are used in the side seams from the armholes to the hips, which then release into pleats to the hemline. Stitch across the top of each pleat (as shown) to secure the pleats. Armholes and neckline should be finished with narrow seams.
And that's it! With the exception of the side pleats, the plain foundation slip looks very similar to slips of later decades. Next in this series will be the coat-dress foundation slip!