Thin, dark line extending from an object that show the limits of a dimension line.
extension line
The thin, dark line extending from an object that shows the limits of a dimension line is called an extension line. An extension line is a thin line, typically drawn at a 90-degree angle to the dimension line, that extends from the object being dimensioned to the dimension line. Its purpose is to indicate the precise points on the object being dimensioned to which the dimension line applies.
Typically, there are two extension lines for each dimension line, one on either end, with arrows at the end of the lines pointing toward the object being dimensioned. The extension lines are usually offset from the object being dimensioned by a short distance, and they do not touch or overlap the object. Additionally, extension lines should be of the same length on both sides of the object being dimensioned.
It is important to draw extension lines accurately in order to ensure that the dimensions being measured are clear, complete, and easy to understand. Extension lines should be drawn exactly to the edges of the object being dimensioned, with no gaps or overlaps, and should be clean and crisp. Using extension lines, in combination with dimension lines, is a fundamental part of technical drawing and engineering design, and is essential for clear communication of information related to size, shape, and orientation of objects that are being designed or manufactured.
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