The Double-Ended Hex Bit is, as many great ideas are very simple. It is a Standard 1/4? hex at one end and 5/16? (the two most popular sizes) at the other but the things that make it so special is that either end will fit a standard battery powered drill. It is color coded on each end, making it easy tell which size you are using at the time and when it is time to change just flip it over.