The inside tubes are interchangeable. This allows you to change from one inside tube to another if you choose. For example, you could change from a 3 inch drop to a 4 inch drop or a 3 inch to a 4 inch or a standard up and down post to a multiposition post just by changing the inside tube. You would need to purchase the other inside tube. The change takes about 30 seconds to perform and is very easy.