The common installation of toilet seats involves positioning the seat on top of the toilet bowl, with the holes in the rear of the seat over the corresponding hinge holes in the top of the bowl. In older seats, metal bolts are inserted through the holes and into the seat, down through the holes in the bowl. From underneath, nuts are threaded onto the bolts. The wrench would be necessary to loosen these nuts from underneath.
The Right Wrench
Because there is no way of knowing what particular size the nuts may be on a particular toilet seat, the best choice is a crescent wrench, also commonly called an adjustable wrench. This wrench has smooth jaws, with a lower jaw that can be moved closer to or farther away from the top jaw. To do this, you twist the gear directly below the jaws. Hold the bolt steady from above, using a screwdriver, to avoid turning the bolt while you loosen or tighten the nut with the wrench.
Toilet Seats That Don't Need a Wrench
Newer toilet seats bypass the need to use a wrench for installation or removal of the seat by using plastic bolts and nuts. To install or remove the nut, you can simply hold the nut with one hand while you turn the bolt from above with a screwdriver. Be sure you do not over-tighten when installing these plastic nuts.
Toilet Seat Complications
If the nuts or bolts on the toilet seat are corroded, additional tools besides the wrench may be needed. If you encounter this problem, use a hacksaw to cut through a nut. You can also cut through the bolt at a point between the stubborn nut and the underside of the toilet bowl, using the hacksaw. Take care not to scratch or damage the toilet.