By activating the "linear dimension" , on the command line you are asked to specify the first point of extension line:
Specify first extension line origin or <select object>:
In practice, (with snap on), we need to select the two vertices of a line or an object that we want to apply a dimension.
Clicked the mouse on the first point, then we are immediately prompted to insert the second:
Specify second extension line origin:
Added second point we will see the dimension line temporarily following the movements of the mouse to be positioned on either side of the line measured.
We'll click a third time with the mouse to define the final position of the dimension line and come out at once by the command.
If after we click the right button will reactivate the last command, to insert another dimension to the drawing.
With the “linear dimension” tool, can be draw dimensions that are only horizontal or vertical, and then measure objects only on its vertical or horizontal projection.
The “linear dimension” always work aligned with UCS axis (X,Y).