A shift is what we'd call a 'slip' in the US. The shift lay next to the skin and protected the wearer from the roughness of the outer garments . It protected the expensive outer garments from the body. It was cheaper to replace than the outer clothing, and the shift was washable.
The shift, for all of the Eighteenth Century is a simple garment, cut loose, straight, and ungathered, going to about the knee. It closed at the neck with a drawstring or was bound with a band.
Here to the right is an extant shift in the Boston Museum of Fine Arts. Drawstring neck. The little ruffles on the sleeve, IMO, would have been intended to show beneath a tight-fitting sleeve on the dress or caraco. This is made in England or France, late C18 to early C19.
The early C18 shift might be somewhat fuller than this example above. After 1785-ish, when the round gown and dresses of thin fabric became popular, the shift began to be cut closer to the body so as not to disturb the line of the dress -- something that wasn't a problem with the robe à l'Anglais or the simple jupe and caraco. The sleeves of the shift, which had been longer and fuller in the first half of the Eighteenth Century, became close fitting or short.
Here's an example of a modern reproduction mid-C18 shift being worn.
A difference between how we regard underwear in C21 and Eighteenth Century is that a shift -- underwear -- was often intended to be seen. It was meant to show beneath the jacket or vest or the neckline of the dress.
See , to the right.
I wouldn't go to the stake on the details of this. It looks fairly fancy, compared to the extant examples. Anyhow, what you got here is a C18 chemise being worn over front-fastening stays, and showing at the top.
(The opening scene of the movie, btw, is her getting dressed in petticoat, caraco and jupe, which is interesting to watch. Reminds me of the dressing scene in Les Liaisons Dangereuses. )
The neckline of the dress or caraco was cut low by our standards. This means the shift underneath was cut low too. The neckline of the shift tended to come just an inch or so above the stays, as it does in the picture above.
This left the heavy work of concealing the bosom to the fichu.
The shift wasn't a full-length garment down trailing in the mud and showing lace at the bottom when the dress blew up in the wind. A petticoat or underskirt might do that, but not the shift. The shift was short. You wouldn't see a shift if you raised your skirts some to step over a puddle.
Below we got us some delightful pictures of an extant shift from Vintage Textile which is at home here. This particular shift is listed as 1820 to 1830, but it's similar to what you would have seen in 1790s.
Our chemise is fashioned from coarse linen and is completely hand sewn.
With time and multiple washings, the linen has whitened and softened. The neckline and sleeve edges are trimmed with hand-embroidered scallops. The chemise has a hand-embroidered, monogrammed "AF" in front.
Chemises, particularly in pattern catalogues, are picture flat so that you can see how they are cut. The triangular side panels in the flat pictures give the impression that the chemise stands out from the body on the side. In fact these side panels push the front and back into graceful bias folds.
A late-C18-to-early-C19 shift would be made of linen or cotton. In 1794, it would probably linen for the middle and lower class. Good quality cotton was still a luxury material in France.
There was a slow movment towards cotton for underclothing as cotton got cheaper in the early C19. Perception of linen in shirts and underwear changed. By the time we get to Beau Brummel ... linen was a more upperclass/stylish fabric than cotton. In 1794, linen would probably have been the fabric of choice, but without the same class
The English word 'shift' fell into disfavor after 1800. 'Shift' became regarded as old-fashioned and somewhat coarse. The snazzier French 'chemise' sneaked in to take its place. That's what we did in English for a century or two ... tossed out perfectly good English words and invited French hussies in.
WARNING: Partial nudity below.
Women's nightclothes closely followed the design of the shift. In fact, one could be used for the other, pretty much.
There are differences The examples of nightshifts I've come across seem to have 3/4 or long sleeves and the shifts of late C18 don't. The nightshifts are often mid-calf length or longer, rather than more knee-ish. So the nightshifts were a specialized garment, similar to but not identical with the shift.
Note the low neckline on these nightshifts below. If that neck isn't tied up carefully with its ribbon or drawstring, the breasts get loose and go showing themselves.
The nightshifts are longer than the shift, but still only mid-calf length. You heroine wouldn't need to hold her hem up as she crept down the stairs, trembling, with a candle in her hand, investigating the noise.
Considering England's -- or France's -- climate, your heroine would be an idiot if she didn't put on some kind of a robe or peignoir so she don't get all clammy and freezing even before the villain has a chance to kidnap her.
The hero seeing your heroine in her nightshift takes on a whole new meaning when you stop picturing it as a floor-length, high-necked Victorian nightdress with 57 little pearl buttons up the front.
This is Baudouin, Le Lever. A bit before our period.
Wheatly gives us Mrs Wheatly Asleep. Close to the 1790s I think. The night cap Mrs. Wheatly is wearing -- see how fancy -- is part of the whole going-to-sleep ensemble.
And Boucher's famous L'Odalisque is wearing a 1745 shift. It's here, click at the site for a closer view. See the low neckline, mid-length sleeves, and the not-too-long length.
Admittedly, these particular nightshifts are painted largely for an excuse to show off skin, but it does look like nightshifts dipped low at the neck.
Night shifts, like shifts, were white or just off-white, made of cotton and linen.
Not silk. Sorry.