Previously
Evidently each embroiderer was allowed one hand on the robe, and two were left handed. I would have trouble working without the other hand to hold the fabric, but perhaps the velvet was weighted.
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I am not an embroiderer but I assumed the other hand is underneath, catches the needle, then pushes it back up again a micro stitch away.
It must be a very well developed talent to do that work - to end up with a top class result!