3-Point Angle Dimension
The 3-Point Angle Dimension 
tool measures an angle defined by three points: a vertex and two points that set
the extents of the angle.
Creating a 3-point angle dimension
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Select the 3-Point Angle Dimension tool.
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Click the vertex point at which the angle is measured.
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Click a point defining the base of the angle.
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Click a point defining the angle.
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Move the mouse to position the dimension arc, 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.