The Modular Step Fitting for dust collection actually pulls apart to reveal 4 separate fittings! The end is 4" which allows a standard 4" hose to be easily attached. Each of the fittings have two different I.D.'s and two different O.D.'s. This gives you a total of 8 fittings in one. This also means that odds are you are going to find the ideal fit for your power tool or machine. When not in use, the fittings can be neatly stacked and stored for easy access when needed. The second image shows the approximate sizes for all 4 sections of the fittings. The three smaller sections are slightly tapered on each end and the largest section is straight on both ends. The taper ends on the 3 smaller section give you more flexibility for fitting those odd sized power tool dust ports. Note: All sizes are approximate