The size is the inside diameter of the rings, which should fits the outside diameter of your telescope tube. Sold in set of two rings. (except 182mm, 235mm, 288mm) For 182mm pairs, press HERE For 235mm pairs, press HERE For 288mm pairs, press HERE Upper side with 1/4-20 thread hole. Lower side with M6 thread hole for dovetail rails, mounting bolts and washers included. Tube rings are powder-coated white except for the 90mm and 140mm rings which are black. This cast-aluminum tube ring set features quick-release, hinged clamp knobs and felt lining, allows you to securely attach a telescope optical tube to a mount and tripod. Designed for Sky-Watcher telescopes and fit some other brands (Orion, Celestron, etc.) of OTAs as well. The hinges open to allow easy installation of the optical tube and the locks can be loosened during use to allow tube Rotation and balancing. In order for astrophotography lovers to capture the spectacular views of the sky, one of the two rings in the set has a 1/4-20 piggyback camera adapter on it, the other has a threaded 1/4-20 hole. The piggyback camera adapter can be removed for mounting an adapter plate for a guide scope on top of the tube rings. Pls choose the size that matches the outside diameter of your telescope tube.