A plumb bob is a weighted object connected to a string that can be used to establish a vertical line. That weighted object can be as simple as a rock or as elaborate as the brass plumb bob shown. Some are bullet- or cone-shaped, but in the DIY home improvement world they’re often in the guise of a chalk box or chalk line.
Plumb bobs have been around a long time. Egyptians used them to build the pyramids and builders throughout the ages have used them to determine if walls are “plumb” or perfectly vertical. They’re ideal for tasks like centering lights over a kitchen island, determining the proper spacing of recessed lights or framing basement walls.
Most of us call Robertson screws “square drive” or “socket head” screws. They were invented by Canadian traveling salesman Peter Robertson around 1906. Up until that time, almost all screws were “straight drive” or “slot head” screws. Robertson’s screws were superior to slot head screws since the square recess automatically centered the screwdriver, provided more surface area for the driver to press against and reduced the chance of slippage.
According to legend, Henry Ford discovered using Robertson screws could shave almost two hours off the assembly time of a vehicle. But since Robertson refused to sell exclusive rights to the screw to Ford, he turned to using a screw developed by a guy named—you guessed it—Phillips.