Most homes have 1 1/2 and 2 inch vents with one larger 3 inch clean out vent. You can recognize the larger one, it sticks out like a soar thumb. So when ordering, remember Size A fits the 1 1/2 and 2 inch vents, B fits the 3 inch vent. If you are one of the rare homes that has an actual 4 inch vent, which is huge, we do not make a product for this. A 4 inch vent has a 5 inch flashing, so it looks like a smoke stack up there.