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Negative Dimensions.
The – sign appears if the coordinate of a dimension is negative.
Generally, do not delete the – sign, but edit the dimension in the normal way.
More Details
Lines have an XY coordinatesystem. •The 0,0 of a Line is at its startPoint •The +Xaxis of a Line is from its startPoint(1) to its endPoint(2) •The +Yaxis of a Line is at +90º from Line. When you add a dimension that is a perpendiculardistance from the Line, the: •dimension is Positive if in +yaxis half of the Line (blue area in the image) •dimension is Negative if in the –yaxis of the Line (green area in the image) In the top image: •Dimensions from the Line(1 to 2) to Points a and b are positive •Dimensions from the Line to Points c and d are negative 

The X & Yaxes of a Part also have their own coordinatesystem. The bottom image to the left shows the CoordinateSystem of the Yaxis The local +xaxis of the Yaxis of a Part is along the Yaxis! In the bottom image: A dimension from the from Yaxis to Point (a or b) in the Blue area is Positive. A dimension from the Yaxis to a Point (c or d) in the Green area is negative. 