LR and SR refer to the centerline radius of the elbow. LR stands for a long radius, and SR stands for a short radius. A long-radius 45-degree elbow has a larger centerline radius than a short-radius 45-degree elbow. The long-radius elbow provides less frictional resistance to the flow of fluids and gases and is therefore used more commonly than the short-radius elbow. However, the short-radius elbow is more compact and easier to install in tight spaces.