All of our flat back earrings come with a 6mm back (this is the most common size and works for most ears). If you need a slightly different length or a replacement, we've got you covered! These backs are compatible with all Aiti flat back earrings. Sold as a single back. * Post width: 18 GA (1mm) * Post Length: 5mm, 6mm or 8mm with a 4mm disc/back * Materials: ASTM F136 implant-grade titanium, 18k gold PVD plated * Waterproof, tarnish-free & hypoallergenic