Phillips

Phillips

The classic type of cross-head screw and screwdriver – so named because the blade of the screwdriver has an "x" shape.

To quote http://www.bodgesoc.org/fastners.faq:

Cross head screws. These come in two varieties, Phillips and Posidrive. The differences lie in the way that the drivers are machined. The Phillips driver has 4 simple slots cut out of it, whereas in the case of the Posidrive each slot is the result of two machining processes at right angles. The result of this is that the arms of the cross are parallel sided in the case of Posidrive, and tapered in the case of Phillips. Posidrive is less likely to slip out because of this, but as there are small protrusions left in the bottoms of the grooves a posidrive driver will not fit a Phillips screw correctly. Both varieties come in a range of sizes.