Segment Dimension
The Segment Dimension 
tool dimensions
the length of a single line segment in one step, without picking its end points
individually.
Creating a segment dimension
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Select the Segment Dimension tool.
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Click the line segment to dimension.
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Move the mouse to position the dimension line, then click to place it.
The Text and Tolerance input boxes let you override the dimension text and add a tolerance. See the section overview for the options shared by all dimension tools.