Tangent to the main discussion:
When sewing a dart, back-tacking at the point puts a lump exactly where you don't want one.
So after stitching off the point of a dart, keep stitching for an inch or so, keeping the fold of the fabric under half the foot, but missing it with the needle. This twists the two threads together into a cord. When you break off, leave enough thread to tie into a knot. Tie a half-hitch in the thread ends, then put a pin through the knot into the point of the dart. Tighten the knot around the pin, so that it ends up right against the point of the dart. Trim the ends to half an inch. The half inch will be neatly twisted into a cord. (But from the right side, it really doesn't matter how the ends look.)
One way to tie a half hitch: form the twisted cord into a loop near the point of the dart, with the thread ends passing under the loop. Use a pin to pull the ends up through the loop. Tighten around the pin.
Joy Beeson