Kind of revisiting this subject from a diatribe post awhile back.
1. What in the heck is a felled seam?
A felled seam is a seam with a self-covered edge. Instead of using bias tape the edge is folder under and stitched down.
2. How many types of felled seams are there?
Who knows for sure, I know of at least six myself.
3. How many lines of stitching are in there in a felled seam.
Anywhere from one, to a lot. The wider the seam and the more stitches the stronger it is. Sails on Clipper Ships had a lot of lines of stitching. Parachutes, people carrying balloon, fabric covered aircraft, etc. (that is anywhere someones life may depend on it usually have an French or English felled seam with 2, 3, or 4 lines of stitching.
4. Ok, I just used the terms French or English Felled Seam. What do they mean.
A French Felled Seam has on edge folded under the other in an interlocking way with the stitching going through all four layers of fabric. An additional advantage of the French Felled Seam is that if cotton or cotton wrapped thread is used in cotton canvas the seam becomes pretty much water tight, because the fabric shrinks around the needle holes and the thread expands in them when it gets wet.
An English Felled Seam, sometimes called a Folded Felled Seam, has the edges folded under and stitched through to the other panel, they do not overlap like the French Felled Seam, there for the stitching only goes through 3 layers of fabric..
Those were the two types of seams used for sewing together large panels of fabric, like canvas, for sails, tents, balloons, and the like.
An Overlapping Seam is a third possibility. With some types of modern fabric whose edges will not unravel or fray a simple multi-stitched overlapping seam can be used.
It gets confusing because there are many seams with the word French in their name, and many kinds of felled and semi-felled seams, and many people use the words interchangeably. Many of them are decorative rather than structural seams.