What is a nipple driver and would I use one to replace a broken spoke? I have not yet built a wheel (or even replaced a broken spoke), so it's not quite clear to me what a nipple driver is. I also didn't find any reference on Sheldon Brown's Wheelbuilding page. What is a nipple driver and how is it used? Is it a necessity or a convenience? Would I use one when replacing a broken spoke?

While a conventional nipple wrench turns the nipple by the exposed flat surfaces that protrude from the rim, a nipple driver turns the nipple from the inside of the wheel.To replace a single spoke you can use a spoke wrench or a driver. Some people find it easier starting the nipple onto the spoke with a driver than with a wrench. If you were truing a wheel the wrench is easier as the tire and tube need not be removed. If you remove the tire, tube and the rim strip you will see the head of the nipple. If you are building a wheel it is more efficient to use the driver to start each nipple on to the spoke. To replace a single spoke you can use a spoke wrench or a driver.