CLR stands for “centerline radius” and is stated as a factor of the diameter of the elbow. Think in terms of a circle; the rate at which the circle is turning is called the radius – smaller and tighter circles have smaller radii, larger circles have bigger radii. If you cut a circle in four equal parts (each part having 90º), the distance traveled around the circle by one of those parts is called the arc. In ducting elbows, the arc AND radius are stated as ratios of the diameter. So, a 4″ 90º elbow with 1.5 CLR will have an “arc” (down the center of the elbow) of 6″ (4 x 1.5 = 6) – the elbow is approximately 6″ long. The same elbow with a 2.5 CLR will have an arc of 10″ (4 x 2.5 = 10) – this 4″ 90º elbow is 10″ long. What this also means is that the “sweep” or severity of the bend is tighter on the smaller CLR elbow.