A sewing machine will put down a metre of stitching in about eight seconds. By hand, that same metre takes roughly forty minutes. We do it by hand anyway, and the reason is not romance.
Lock stitch versus saddle stitch
A machine makes a lock stitch: one thread from above, one from below, knotted between the layers. It is fast and strong until a single stitch is cut — at which point the knot releases and the seam runs like a ladder in a stocking.
A saddle stitch uses one thread with a needle at each end. Both needles pass through the same hole from opposite sides, so every hole contains two threads crossing. Cut one stitch and the two on either side hold it closed. You can cut three in a row and the seam still will not run.
Why nine
Stitches per inch is a trade-off. Too few and the seam gapes under load and the thread sits proud where it can abrade. Too many and you have punched so many holes so close together that the leather itself becomes the weak point — you have effectively perforated it, like a stamp.
Nine to the inch on 3.4 mm shoulder puts the holes far enough apart that the fibre between them is still doing work, and close enough that the seam pulls up tight and flat. On thinner calf we go to eleven; on a 4 mm belt, seven.
The awl is the whole skill
Pricking irons mark where the holes go, but they do not open them. A diamond awl does that, and it has to enter at a consistent angle every single time. Get the angle wrong and the stitch line on the back looks drunk even though the front looks perfect. This is the part that took us three years to get reliably right and the part that instantly identifies hand work to anyone who knows what to look for.
Thread
Waxed linen, 0.45 mm for wallets and 0.6 mm for bags. Linen is weaker than polyester in a tensile test, which sounds like an argument against it — but linen abrades at roughly the same rate as leather, so the seam ages as one object. Polyester outlasts the leather and eventually saws through it.
What it costs you
Roughly two extra days on a satchel. That is most of the reason a hand-stitched bag costs what it does, and the only honest justification is that the seam will still be there in thirty years. Everything else is decoration.
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