A chemisette was a false front for below the neckline of an antique dress. They were worn in Victorian and Edwardian times. A chemisette allowed a woman to wear a low necked dress or bodice, but to add some lace trim or a collar to the neckline both to change the look of a garment for variety and for reasons of modesty. A chemisette was also known as a guimpe.
A chemisette or guimpe does not usually have side seams, but may just slip over the head and tie around the body beneath the bust. Some chemisettes do have side seams, but they are all short, just long enough to cover the bust and neck area.
See my blog post about antique underwear: