What distinguishes the symbol for a melt-through weld from an all-around fillet weld?

The distinguishing feature of the melt-through weld symbol is that the circle used in the symbol is filled in. This filled-in circle indicates that the weld is intended to penetrate completely through the base metal, which is a key characteristic of a melt-through weld. It visually communicates to the welder that full fusion is required, ensuring that the weld will bond effectively with the base materials on both sides.

In contrast, the all-around fillet weld typically features an unfilled circle, signifying a different welding requirement. Understanding this visual cue is crucial for interpreting welding symbols correctly and achieving the desired weld quality.