The fabric is a lovely cream-colored sueded silk and those very neat rows of double stitches are created by using a set of twin needles both thread with the same color thread. The bobbin thread zig-zags between the two rows of stitches and applies a small amount of tension to create the little ridge.
These needles come in different sizes and widths:
In the photo below you can see the twin needle positioned in the needle shaft. They are ingenious little things, two needles fit into a bar (the red thing in the picture) which has a single shank to fit into the needle shaft.
In these pictures I'm working on another shirt in violet-colored silk dupioni. Above is the stitching around the cuff edge and below shows two rows of twin stitches across the tops of the pockets. The thing about the twin needle that makes it so enjoyable to use is it gives a very even, perfect, professional look with very little effort. Just be sure to sew more slowly than you might normally. Less chance of the extra thread getting all tangled up that way.
While I was working on my two new shirts I dug out the shirt below and harvested the buttons. I have no idea how old this shirt is, at least 25 years. It is old silk that was once pure white but has mellowed to a soft pearl color. It's in pretty bad shape, it was lovingly worn for many years, but I loved the buttons so much I couldn't bear to throw it away. So I cut the pretty mother of pearl buttons off, there are 10 of them, and saved them for another project.So it was a pleasant and productive weekend. I'm really delighted with the two new silk shirts and when they are finished I'll post pictures. Recently a friend commented that I was the last person he knows who knows how to sew. I had to chuckle - clearly he doesn't read internet sewing blogs, there are hundreds of them. Nothing is more satisfying than making a beautiful garment and having someone stop me ans say, "Wow, where did you get that? It is gorgeous."
Thanks for reading.