A ladder stitch — also called an invisible or slip stitch — joins two folded edges from the outside while keeping the thread hidden inside the seam.
How it works
Take a small stitch along the fold of one edge, then a small stitch directly opposite along the fold of the other, alternating back and forth. The working thread crosses between them like the rungs of a ladder. Every few stitches, pull gently and the rungs disappear inside, drawing the folds together with no thread showing.
Why it exists
Some seams cannot be sewn inside out. Closing a stuffed body, attaching a head to a neck, joining a limb after it has been filled — in each case the piece is already three-dimensional and the seam has to be worked from outside. A ladder stitch is how you do that without the stitching showing.
Ladder or whip?
Whip stitchA whip stitch is a hand stitch worked over the edge of two p... for fleeceFleece is the knit fabric used as the outer skin on most mod... More seams where pile will close over the thread. Ladder stitch for smooth fabrics with no pile to hide behind, and for anything already stuffed. On a puppet you will use both, often on the same build.
Keep the stitches small and directly opposite
Offset stitches make the seam zigzag once it is pulled closed, and that shows.
Do not over-tighten
Pull just until the folds meet. Pulling harder puckers the seam and the pucker is permanent.
The hack: pull the thread closed every four or five stitches rather than at the end. A long run of open rungs tangles, and closing gradually keeps tension even along the whole seam.