As shown in the following illustration, you can create dimensions by selecting an object to dimension (1) and specifying the dimension line location (2).
You can also create dimensions by specifying the extension line origins. By default, for lines, polyline segments, and arcs, extension lines originate at the endpoints. For circles, the endpoints of the diameter are used at the specified angle.
Creating Linear Dimensions
Creating Radial Dimensions
Creating Angular Dimensions
Creating Ordinate Dimensions