The Boeing 737-900ER is the largest 737 Next Generation variant and the one Boeing built specifically to compete with the Airbus A321. The ER designation, which stands for Extended Range, introduced auxiliary fuel tanks and structural reinforcements that allowed the aircraft to carry a full load of passengers further than the base 737-900. United Airlines and Delta Air Lines became major operators, using the aircraft on high-density domestic transcontinental routes. The 737-900ER competed reasonably well against the A321 on capacity alone but lost the comparison on range, where the A321neo substantially outperformed it. The 737 MAX 9 and MAX 10 have since taken over the role, and 737-900ER production ended in 2019.