Generally, women would wear a shift (English) chemise (French), or sifft (Welsh) made of flannel, cotton or linen under their gowns. They may have been made in a simple T shape like a man’s smock but if this was so, some had a low-cut top since they are not shown in the illustrations where the bedgown was low-cut and no shawl or kerchief was worn around the neck.
Only one reference to underwear has been found in the literature, and this by a Welshman (who had more reason to know what Welsh women wore under their bedgowns than an English visitor). Pritchard, in 1828, advised the women of Pembrokeshire and elsewhere in Wales ‘to throw off their flannel shifts, and wear linen ones’. (T. J. Llewelyn Prichard, The Adventures and Vagaries of Twm Shon Catti; (1828), p. 46)
Anne Clayton [a pauper?] was given two flannel shifts by the parish. (Vestry book of Llanfihangel y Creuddyn, 1791)
J.C. Ibbetson’s watercolour of washerwomen at Llanwrst in 1792 shows some of the women with the front part of their gowns unfastened, revealing a white v-neck shift. (‘Washerwomen near Llanrwst Bridge’/ ‘The Ususal spot near Llanrwst bridge over the Conway River, where the Washerwomen carry on their work’. NLW PB8558)
In his description of the wool industry of Montgomeryshire in 1799, Walter Davies praises the quality of the flannels which had a fine nap, making them particularly soft and ‘rendered them exceedingly well adapted to be worn next to the skin of the most delicate invalid’. (Davies, Walter, General View of the Agriculture and Domestic Economy of North Wales, (London, 1810), p. 394)