The two needles are attached to a single crossbar. Stitching two rows simultaneously, they create decorative effects for heirloom sewing, pintucks, fancy hems, and other decorative effects.
The two numbers in the size of the needle indicate the space between the tips and the size of the needles, respectively. For instance, a 3.0mm/80 needle is two size 80 needles spaced 3mm apart.
For most woven fabrics, and some knits and synthetics. Slight ballpoint.