When is a white dress not a white dress? When it's a chemise!
Above is shown the only illustration of a women’s undergarment that I've seen in many, many illustrations (this was found by David Rayner). We can't be sure this is not a dress, but it seems probable. The features of this garment are consistent with the features of the garment worn under a dress in illustrations - tight slightly wrinkled sleeves to the wrist, folds of garment at the feet, a keyhole neckline tight about the neck.
The garment appears to be a tunic, tightly fitted to the forearms, looser on the upper arms and body, with a keyhole neckline. This gives us a general layout for a women’s chemise, (although a noble ladies chemise may differ a little from a nuns), but does not give us any construction details.
Unfortunately no female 12th century garments exist, and Romanesque artwork glosses over fine details such as seams. Seeing that this differs little in shape (only length) from male garments, we must turn to them for details. The extant men’s garments are mostly clerical garments, but are expected to differ in only small details of construction from secular garments. Indeed a great many clerical albs feature sleeves which are much tighter in the lower arm than upper arm, very full of skirt, and the sleeve length and garment length are often much greater than seems practical for a human body. Keyhole necklines exist, but other forms of neckline are very common. There seem to be a great many parallels between those garments and this illustration, so this does suggest that albs are a suitable source of inspiration for chemises, if used with care.
One final thing this illustration has taught me concerns kilting up the skirts of a chemise. I've tried kilting up the skirts of my longest chemise at the waist. It's a quite effective way of shortening a chemise. I put on a belt, then pull a large fold of the chemise over the belt. I then get a friend to help pull the chemise down to my desired length. The chemise can be ankle of floor length, longer in back, whatever length I want. It seldom falls down, only under duress of the most vigorous dancing. I find this an extremely practical way to adjust my chemise length. The only requirement is that there is enough fabric to fold up, ie that the chemise is at least say 10cm longer than the longest belted length you want it to be. The only inconvenience of this technique is an added bulge at the hips caused by the chemise - if the garment worm over the chemise isn't kilted itself, this can look a little silly to modern eyes. My looser Austrian dress is not so tight that this is a problem, and stiff enough to ignore such effects though, so it works perfectly with this technique.