A combination ER wrench is disclosed for use in installing and tightening collet holders in connection with milling machines or machining centers. The disclosed wrench combines the function of multiple wrenches. The disclosed wrench has two open ends and two box ends that fit various collets and collet holders common in the industry, along with a box end to fit a retention knob. The open ends of the wrench have arcurate jaws and five points to contact the collet and/or collet nut. The box ends of the wrench are openings shaped to contact the collect and/or collet nut.