Lace tulle, a fine net, with some elastane, and although described as 'stretch', the embroidery is not stretchy, so this limits how much the background lace can stretch. Paired edge, so you get both directions to make symmetrical sides. This gives a really professional look to the top cup on a bra, or can of course, be used on all lingerie where symmetrical sides are desired.

The wider lace means its also often possible to cut the bottom cups out of the plainer areas.