What is the special wrench designed to reach the hex jamnut beneath a fixture called?

The special wrench designed to reach the hex jamnut beneath a fixture is known as a basin wrench. This tool has a unique design featuring a long handle with a spring-loaded jaw that allows it to grip and turn nuts located in hard-to-reach areas, such as those found under sinks. The basin wrench's ability to navigate tight spaces makes it invaluable for plumbing tasks, particularly when installing or repairing faucets or other fixtures where access is limited.

While the other wrenches mentioned have their own specific uses—such as the pipe wrench for gripping and turning pipe sections, the box wrench for providing a secure grip on fasteners, and the adjustable wrench for accommodating various sizes of nuts and bolts—they do not possess the same specialized design for working in confined spaces, which is where the basin wrench excels. This characteristic of the basin wrench makes it essential for effectively managing plumbing installations and repairs.