The edge block is a block of metal (steel) placed at both end of the welded groove to prevent over exposing the radiographic film at the end of the weld. Over exposure "burns" the film turning the film white at the ends of the welded groove. Discontinuities in those areas would not be detected as a result.

Weld tabs are extensions of the weld groove to ensure sound weld is deposited at the ends of the welded groove. They are also referred to as extension tabs or run-off tabs. The weld is initiated beyond the end of the groove to allow the arc to stabilize and develop sufficient heat so that sound weld is deposited by the time the arc reaches the actual groove.

Best regards - Al