Dimensioning distances

You can dimension the distance:

Of a single line. This measures its length.
Between two objects — for example, the distance between two parallel lines or between a line and a circle.
Between two control points. This can be done regardless of whether they belong to one object or to different objects.

Prinect dimensions:

Notes

For distance dimensions, the default dimension line is the direct distance between two control points or objects. While you are dimensioning a horizontal or a vertical offset distance, releasing the SHIFT key before placing the dimension line will restore the direct distance dimensioning mode.
Dimensioning the distance between two lines is possible only for parallel lines. That is, when in the Two Objects dimensioning mode, clicking consecutively any two nonparallel lines — but not their control points — will dimension the angle between them or their imaginary projections, not any distance between them. Nonetheless, you can dimension the distances between any control points that belong to the two lines.
Sometimes there are several ways of dimensioning the same set of objects. For instance, if you are dimensioning the distance between a circle and a (control) point, you can measure, among other objects, the distance between the center of the circle and the point, between the nearest tangent to the circumference and the point, or between the farthest tangent to the circumference and the point, depending on the types of the two objects involved. In such cases, to switch between the ways of dimensioning, press CTRL after you click the two objects.
To display the existing dimensions in the graphical area, right-click a blank space, and then check Show Dimensions on the context menu.
Learn more about how to set general dimensions options.