Select two points or edges to create a horizontal dimension between them, then choose a position for the dimension text. You can choose lines for the dimension, but they are not actually used. Instead, the endpoint nearest your selection is used. The selection location determines which side of a circle or ellipse (or its arc) is used. Splines are not selectable for this function, but their endpoints can be used.

Note that the Dimension measures only the distance in the horizontal (x-axis) direction. Any vertical distance is ignored.