For Chainmaille: Works most often with 19 and 20 gauge jump rings or as a replacement for them in closed ring form. The thickness of this oring usually compensates for the lower AR that should be used when replacing jump rings with EPDM rubber orings.
For leather: Works well with 3mm leather cord or to hold together 2 strands of 1.5mm or 2mm leather cord. Works to create lace patterns with leather.
Other uses: This size works very well as a bead spacer to protect delicate beads that are strung such as lampwork beads. They can also act as a buffer to hold beads in place in wireworking. Orings are used as stitch markers for Knitting & Crochet and can also be used for Macrame and Seed Bead Crafts. EPDM Orings were developed to form a waterproof seal. They can be used with LED lights to form a waterproof seal between the bulb and hardware. These have also been used for body piercings that often require a stopper of this sort, tattoo equipment, paint guns, bicycle & Industrial parts.
Sizing: The ID or Inner Diameter is the size of the hole. Generally, choose an inner diamter slightly smaller than you require. The rubber will stretch to fit over items and form a strong seal or hold items in place.