This pair of two 90mm Vixen Tube Rings clamps onto your optical tube assembly (OTA) and provides a solid and secure mounting point for your dovetail plate.
Each ring is designated by the inside diameter to match the OTA's outside diameter, and the clamping design allows you to put them anywhere along the tube's length to achieve the proper balance or rotate the orientation of the tube to avoid interference with other gear. The inside of the ring has a slip-resistant non-marking material that holds the OTA without damaging it. The bottom of the ring has an M6 threaded mounting hole for you to attach an optional Vixen #2661 dovetail tube plate to mount your OTA to your mount head, while the top of the ring has another threaded M6 hole to accept their #3548 accessory plate so you can easily mount a camera for wide field astrophotography or an autoguiding guide scope.