The interestingly titled monkey wrench is the predecessor to the now much more common adjustable wrench. Monkey wrenches were designed to loosen and tighten hex-shaped bolts via two metal jaws that leverage the bolt on either side.
The size of the opening in the wrench is adjusted with a threaded shaft on the back of the wrench. Once the wrench is open to the proper distance, a locknut on the threaded shaft is tightened, maintaining the distance that the jaws of the wrench is opened.
Over the last century the somewhat bulky monkey wrench has become increasingly rare thanks the adjustable wrench, which has a more compact jaw design and therefore can be used in tighter spaces with less clearance.
The monkey wrench is designed to tighten and loosen any hex-shaped bolt, or any four-sided bolt for that matter. Monkey wrenches were commonly used for household tasks, such as loosening and tightening large plumbing bolts.
This type of wrench is also suitable for tasks such as automotive maintenance and repair, though it works best on larger bolts.
As opposed to the somewhat antiquated monkey wrench, pipe wrenches are still a common tool for household work, in particular plumbing and pipe work. The serrated jaws of a pipe wrench look similar to those of a monkey wrench, but are designed to rock in the frame they are installed.
The jaws of a pipe wrench are usually tightened to a particular width with a pair of knurled thumb nuts that ride on a threaded shaft. When the wrench is opened to the desired distance, the thumb nuts are tightened.
Pipe wrenches are designed to be used on soft piping, for example the copper piping often found in household plumbing. The wrench is placed onto the pipe and then angled against it so that the serrated teeth dig into the metal, allowing the pipe to be easily turned to loosen or tighten it in any fitting that the pipe goes into.
Pipe wrenches are not designed to be used on hard hex nuts, since the teeth would damage and round off the edges of the nut. However, if a nut has already been rounded off, a pipe wrench can often loosen it by gripping onto the damaged nut.
Pipe wrenches come in wide variety of sizes, from smaller than 10 inches to larger then 48 inches.