A dimension line is a thin line running parallel to a particular dimension of an object that describes the parameters of the dimension. It has arrow heads at both ends.
Dimension lines help in the clear and comprehensive description of an object. There are conventions used to eliminate ambiguities in the use of dimension lines. For instance, lines should be placed no closer than 3/8 inch from the outline of the object. They are also often broken in the middle to create space for indicating the value of the dimension. In case space is limited, the arrow heads at the ends of a dimension line can be substituted with a heavy black dot.