Perfect for sewing leggings/swimmers hems and waistband. Optionally used for top stitching bindings (band binding option) for skater dress.
Twinneedles can be used in either size according to your preference, they both perform the same function it is just whatever you prefer visually.
I like narrow (2.5mm) hems for leggings and wide (4.0) hems for waistbands. Same thing for swimmers, I prefer the narrow for leg finishing and wide for waistbands. Thenarrower needlewillgiveless tunneling between stitches on lighter weight fabrics.
Schmetz Needles Stretch Twin 4.0