Think "skirt to be worn with a tunic-length top" and you have the essence of the 1920s tunic-dress foundation slip. This type of foundation slip is needed for what is called a tunic dress (tunic dress example). The slip top is a modification of the camisole top, while the slip bottom is a wide band of the dress fabric deep enough to prevent the upper part of the foundation slip from showing when worn.
To create a tunic-dress foundation slip, start with a plain slip pattern with a camisole top. Take in excess fullness at the hip line with darts (as at a in the image below). Use French seams under the arms and finish the neckline with lace edging. Cut the slip at the length the dress fabric is to be attached. Sew the dress fabric to slip and hem. Voila! You have a tunic-dress foundation slip.
And this concludes my series on 1920s foundation garments! I hope you found it informative and useful. :)