Torx is taking over. A lot of the contemporary components are switching over to torx hardware rather than the standard metric hex head. Boo! More tools to buy, but this one at least saves you a bit of extra cash-money. The most common torx sizes that I encounter are t10 and t25. T10 is common on hydraulic brake systems, often the bleed port screws are t10's. And t25 is the torx equivalent of something between a M5 an an M4 hex, so that's often found on stems, caliper bolts, seatposts, etc.