Perfect for sewing leggings/swimmers hems and waistband. Optionally used for top stitching bindings (band binding option) for skater dress.
Twin needles 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. The narrower needle will give less tunneling between stitches on lighter weight fabrics.
