Extension and dimension lines are never used in conjunction with a callout for welds.

Extension Lines

Extension lines are lines drawn on a blueprint or sketch that extend from the outline of a part to call out dimensions or indicate a detail’s location.

Dimension Lines

Lines used in a drawing to show the direction and extent of a dimension, often terminating in arrowheads that point precisely to the dimension's extent.

Callout

An annotation on a drawing, usually detailing information of a specific feature, dimension, or instruction.