Experimental optimization of tab and slot plug welding ...
The backing bar symbol has the same shape as the plug or slot weld symbol, but context should always make the symbol's intention clear.
Welding Joints, Positions, and Symbols - ppt video online download
A weld symbol represents a particular type of weld (groove welding, fillet welding, plug welding, etc.) and always forms a part of the welding symbol drawn on a fabrication drawing.
Basic Welding Symbols Their Charts and Drawings
Weld symbols are used to indicate the welding processes used in metal joining operations, whether the weld is localized or “all around”, whether it is a shop or field weld, and the contour of welds.