Waxed thread

Waxed thread is thread, usually polyester, nylon, or linen, coated with a layer of wax that thickens each strand slightly, helps it grip inside a stitch hole, and adds some resistance to moisture and abrasion. It comes pre-waxed from most suppliers, though some crafters still wax their own thread with a block of beeswax.

The wax coating is what keeps saddle stitches tight and locked in place, since it creates friction against the leather inside each hole rather than letting the thread slide loosely. Linen thread tends to need more frequent hand waxing and stretches less, while polyester holds up better to sunlight and repeated flexing, which is why polyester has become the more common modern choice for everyday goods.

Pick a thread thickness that matches your needle and stitch hole size, since thread that is too thick will not pull through cleanly and thread too thin leaves weak, loose looking stitches. A thread conditioner or extra wax pass helps if a spool feels dry or fuzzy. A common mistake is using unwaxed sewing thread meant for fabric, which frays and weakens quickly under the friction of hand stitching leather.