I realize that it might be hard to picture what the sewing book looks like. There is old-fashioned handwriting, homespun fabric samples, and crumbling, acidified pages to look at. Right now I am a bit overwhelmed by my Winter semester at Hunter, but I had always envisioned including pictures of the book with my posts. When I get a little time, I will start to take pictures of the pages and post them too.
(Two different fabric samples with examples of French seams. Maybe she made a running stitch on the right side of the fabric in the first process, and then folded it in half with the wrong side showing and made a running stitch next to first line of stitches in the second process. Then it looks like she opened the fabric back up to the wrong side of the fabric and pressed the seam to the side so that the seam isn't visible on the right side of the fabric. Sorry, I'm just guessing from the samples and being lazy by not looking it up.)
French seam is a double seam, one seam within another.
(Two rectangles have been joined together along one length, with the right side of the fabric facing out. Some of the seam allowance has been trimmed away. I have never sewn a French seam before, so I am not sure how one is sewn. I will have to look it up in one of my sewing books.)
Plain Seam is joining two pieces of material.
(Two small rectangles of fabric are joined together along one side of their length, with the seam in the middle of the sample. The wrong side of the fabric is facing out, except at the seam, which is pressed open and flat.)