Use Attach mode (also know as running snap) when you are drawing lots of entities that need to connect properly.
Click the Attach button to toggle Attach status.
Right click the Attach button to select an attach mode.
When Attach mode is on, you can select a point anywhere on an entity, and the point will snap to significant point on that entity. The significant point can be, for example, the endpoint of the entity. If no entity is found, it behaves as if Attach is off. Attach is ignored when you type coordinates or use a modifier. Attach has precedence over ortho and snap locking.
Toggle the mode on or off with the Attach button on the Button Bar. The default setting is off.
Right click on the Attach button 
to select the active modifier mode. You can also use the ATTACH MODE command (Options > Attach Mode). CC3 defaults to Nearest Endpoint if none is chosen. The dialog box gives you the following choice of modifiers:
· Nearest Endpoint [EPT];
· Center [CEN];
· Midpoint [MID];
· Nearest On [ON].
· Nearest End or Midpoint
· Nearest Fraction - specify in Fractional Denominator (e.g 4 would lock to the nearest quarter)
Right clicking on the Attach button sets the attach mode, it does not turn attach on.
Related Topics
To see more details on how to use the Attach dialog box click here 