Lock-stitch uses an upper and a lower thread which combine – or lock – together as they pass through the leather of a shoe, creating a stronger overall stitch. The heavy-duty lock-stitch seam is used when attaching the sole to the welt, crucially it is the stitching that runs around the outside of the sole (rather than piercing the part under the foot) to maximise the sole’s water-resistance.

The lock-stitch is used because the thread chain won’t unravel if it breaks down at any particular point in the shoe, giving a stronger overall construction.