What tool is primarily used for turning square head nuts and bolts?

The tool that is primarily used for turning square head nuts and bolts is the open-end wrench. This type of wrench features two jaws that are open at one end, allowing the user to slip it over a nut or bolt head for tightening or loosening. The design makes it particularly effective for gripping the flat sides of square head fasteners, providing adequate torque and control.

Open-end wrenches are versatile and can fit various sizes as long as the fastener's shape accommodates the wrench's jaws. This makes them a common choice when working with square head nuts and bolts, as they can be maneuvered easily, particularly in tight spaces.

In contrast, the other options may not provide the same level of effectiveness or suitability for square head fasteners. A square head socket is designed for use with a ratchet and may not be as readily available or practical for all situations. A box wrench typically envelops the head of a bolt entirely, which can limit its use to rounded shapes, while a double-end wrench can be less effective for square fasteners since it may not fit the shape as securely. Thus, the open-end wrench stands out as the correct tool for the task.