An extension line is a thin, dark, solid line that extends from a point on the drawing to which a dimension refers.

By adding dimensions to a technical drawing the drafter is providing information about the size and location of features of a part.

A heavy straight line should be drawn under all dimension values that are to scale.

Typically, all dimension values and notes are lettered horizontally and should read from the bottom of the sheet.

Although an engineer has placed dimensions in specific locations the drafter has the freedom to arrange them as he deems necessary.