Lines have an X-Y coordinate-system.

•The 0,0 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 the Line.

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

•dimension is Positive when the dimension is in +y-axis half of the Line (blue area in the image).

•dimension is Negative when the dimension is 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 are positive

•dimensions from the Line(1 to 2) to Points c and d are negative

The X and Y-axes of a Part also have their own coordinate-system! They are similar to Lines.

The bottom 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!

In the bottom image:

•dimensions from the Y-axis to Points a and b are positive.

•dimensions from the Y-axis to Points c and d are negative.