Selecting boundary edges

It uses the selected objects to define the boundary edges to which to extend an object.

Object to extend

Specifies the objects to extend. Press ENTER to end the command.

Select an object pressing and holding SHIFT to trim

It trims the selected objects to the nearest boundary rather than extending them. This is an easy method to switch between trimming and extending.

Fence

It selects all objects that cross the selection fence.  Selection Fence The selection fence is a series of temporary line segments specified with two or more fence points.  The selection fence does not form a closed loop.

Specify the first fence point: Specify the starting point of the selection fence

Specify next fence point or [Undo] or Enter to confirm: Specify the next point of the selection fence or enter   U

Specify next fence point or [Undo] or Enter to confirm: Specify the next point of the selection fence or enter   U or press ENTER

Example

Selecting objects to extend using Fence (Red line):

Result of the extension::

Edge

Extend the object to the implicit edge of an other object or only to an object that really intersects it.

Example

Selecting a boundary edge and an object to extend:

Enter and edge extension implicit mode [Extende/No Extend] < current>: Enter an option or press ENTER

Extend

It extends the boundary  edge along its natural trajectory, to intersect an other object or relative edge.
In this case the extension of the trimming object (vertical) is intersected.

No Extend

It extends the object only to an other object that intersects it.
In this case there is not any real intersection, thus any extension.

Undo

It reverses the most recent modifications performed with the EXTENDcommand.