The screw-driver denoted by Fig. 418 is a tool for fixing and unfixing small bolts having square heads or hexagonal ones. The tool is fixed in an ordinary handle, and has a boss at one end in which is a recess that fits the heads of the bolts to be fixed. A screwdriver of this form may be made of a sufficient length to enter a deep recess or other opening which is not accessible to arms and other tools. A powerful screw-driver for bolts having grooved heads is shown by Fig. 419 ; this is a convenient tool for entering narrow openings in which a tee-screw-driver cannot be used, and is specially adapted to large screw-heads that are provided with deep grooves to receive the ends of the screw-driver. Each one of the thin ends that enters the screw-heads is at right angles to the other end, for the convenience of rotating screws in places where there is but little space for the handle of the screw-driver to revolve. The tee-screw-driver denoted by Fig. 420 is a powerful tool which is much used, but must be employed directly over the screw, and in a position which is in a straight line with the screw to be rotated.