Click the Smart Dimension command 
.
               
Click a line (A).
Smart Dimension dynamically displays a linear dimension.
Position the dimension, and then click to place it (B).
 
               
Tip:
Before you click to place the dimension, and depending upon the number of elements selected, you can use the following keys to affect the dimension that is created:
| 
                           
                            Keyboard options  | 
                        
                        
                           
                            Result  | 
                        
                     
| 
                           
                            A  | 
                        
                        
                           
                            Changes between a linear dimension and an angular dimension.  | 
                        
                     
| 
                           
                            B  | 
                        
                        
                           
                            For PMI, changes the dimension orientation back to the previous dimension plane in sequence.  | 
                        
                     
| 
                           
                            D  | 
                        
                        
                           
                            Changes between a radial dimension and a diameter dimension.  | 
                        
                     
| 
                           
                            N  | 
                        
                        
                           
                            For PMI, changes the dimension orientation to the next dimension plane in sequence.  | 
                        
                     
| 
                           
                            Shift  | 
                        
                        
                           
                            Alternates between: 
 
  | 
                        
                     
| 
                           
                            Q  | 
                        
                        
                           
                            Turns automatic locate off and on so you can select a different element to dimension to.  |