Combination Box/Open End wrenches are designed for a wide variety of work. Their box opening adds to their versatility and strength. The Flare Nut pattern with its hex box opening is especially useful in air conditioning and refrigeration work where tubing terminates on flare nuts. The thinner and slimmer Electronic pattern is designed for work where access and clearance are limited.
Combination wrenches are among the most versatile hand tools because they integrate two functional designs into a single tool. The open-end jaw, allows you to easily slip the wrench onto a fastener from the side. This makes it ideal for speed and convenience, especially in tight production or service environments where nuts and bolts must be repeatedly repositioned. The opposite end box head fully surrounds the fastener and provides the best grip. The box design distributes force evenly across multiple contact points, reducing the likelihood of stripping nuts or rounding corners. This creates greater torque and improved turning power. Given the choice, always use the box end for best performance. This is particularly true when working with stubborn or corroded fasteners. Box end wrenches, like the one pictured above are offset slightly at the ends, providing knuckle clearance and less chance for worker injury.
Mis-Use/Abuse:
Do not use the open end of these wrenches to free a “frozen” nut or final tighten of a hex nut. You should use the box opening and never use an extension on the wrench handle or use the box end as a hammering device.
Source: Hand Tools Institute and Ampco Safety Tools