4" adj gas vent pipe. 12" length adjusts from 3-1/2" to 10-1/2", 18" length adjusts from 6" to 16". Size refers to the inside diameter of the pipe or fitting. The outside diameter of RV is approximately 1/2" larger. Grade: UL Listed, Temperature Rating: 550 F, Size: 4" x 18", Adjustable Length: 6" - 16", Material: Alum Liner/Galv Steel Jacket No. 5RV-EZAJ12: Grade: UL Listed, Temperature Rating: 550 F, Size: 5" x 12", Adjustable Length: 3-1/2" - 10-1/2", Material: Alum Liner/Galv Steel Jacket No. 5RV-EZAJ18: Grade: UL Listed, Temperature Rating: 550 F, Size: 5" x 18", Adjustable Length: 6" - 16", Material: Alum Liner/Galv Steel Jacket No. 6RV-EZAJ12: Grade: UL Listed, Temperature Rating: 550 F, Size: 6" x 12", Adjustable Length: 3-1/2" - 10-1/2", Material: Alum Liner/Galv Steel Jacket No. 6RV-EZAJ18: Grade: UL Listed, Temperature Rating: 550 F, Size: 6" x 18", Adjustable Length: 6" - 16", Material: Alum Liner/Galv Steel Jacket