The spline of the NRS Oar Right Large fits between the horns of an open oarlock to lock your oar blade in position for the most effective stroke. You can still feather the oar blade by simply tugging the oar in so the spline of the NRS Oar Right Large clears the oarlock horns. For Carlisle and Cataract oars, use the Molded Oar Sleeve (sold separately) with the NRS Oar Right Large for proper fit. Instructions that come with the NRS Oar Right Large say to put a screw (which is provided) through the tip of the spline into the oar shaft. This is only applicable if you're using it on a wooden oar. DO NOT do this on Carlisle or Cataract oars as it can damage the oar shafts. The NRS Oar Right Large adjusts from 2 in to 2 1/4 in (Carlisle & Cataract Oars with Oar Sleeve) and are sold individually.