A french seam is something my mother taught me, that she was taught to do to prevent fabric from fraying before the invention of the overlocker. I now use this method for medieval hand-sewn garments. I don’t actually have any evidence that it is “”period”” or historically accurate, but given that it’s a practical solution to fraying edges I see no reason to think that people in the past wouldn’t have used this technique. It’s elegantly simple. First, with wrong sides facing, pin and sew a running seam about 5mm from the edge of the fabric (approximately! Can vary depending on all sorts). I often sew this seam about 10mm in and then trim the rough edge back, if there is room to do so.