Why is the dimension 'negative' in the Dimension dialog'?

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Why is the dimension 'negative' in the Dimension dialog'?

Why is my dimension negative?

Negative Dimensions.

The sign appears if the coordinate of the dimension is negative.

Generally, do not delete the sign, but edit the dimension in the normal way.

More Details

MD-Negative-DimA

Lines have an X-Y coordinate-system.

The Origin of a Line is at its start-Point

The +X-axis of a Line is from its start-Point(1) to its end-Point(2)

The +Y-axis of a Line is at +90º from Line.

When we add a dimension that is a perpendicular-distance from the Line, the

dimension will be Positive if in +y-axis half of the Line (blue area in the image)

dimension will be Negative if in the -y-axis of the Line (green area in the image)

In the top image:

Dimensions from the Line(1 to 2) to Points a and b will be positive

Dimensions from the Line to Points c and d will be negative

MD-Negative-DimB

The X-AXIS and Y-AXIS of a Part each has its own coordinate-system.

The image to the left shows the Coordinate-System of the Y-AXIS

The local +x-axis of the Y-AXIS of a Part is along the Y-AXIS!

Thus, the local +y-axis of the Y-axis of a Part is at +90º from Y-axis

Thus a dimension that is a perpendicular-distance from the Y-AXIS to a Point is in the green area will be negative ...

... and this is in the +X axis direction of the Part!