Robertson screws Screws (for metal or wood) with a square hole in the top rather than a straight or X-shaped one. Robertson screwdrivers come in different colors to indicate what size they are. Green number ones and red number twos are the most common. Robertson screws are just about impossible to strip, unlike Phillips-head ones, which become unusable about thirty seconds after you've brandished the screwdriver at them. They'd be popular in the States except that Henry Ford wanted exclusive rights to them, and Robertson (the inventor, a Canadian) refused to sell.