What is the difference between a back weld and a backing weld?

• A. A back weld is done after the weld on the other side of the reference line; a backing weld is done before
• B. A back weld is done before; a backing weld is done after
• C. They are the same process
• D. Back weld is only for fillet welds

Answer: (A)

In welding terminology, the order in which you weld from opposite sides matters for how the root of the joint fuses. A backing weld is laid on the backside before you run the weld from the reference side. It provides a stable root and helps ensure penetration when you eventually weld from the front. A back weld, on the other hand, is applied after the weld on the opposite side has been completed—it's the pass you add to finish and strengthen the joint from the far side. So the difference is about when the weld on the backside is placed relative to the other side’s weld.