First Object

Selects the first of two objects required to define a 2D fillet.

If you are working with a 3D model, you can also select the edge of a 3D solid. (Not
available in AutoCAD LT.)

Second object or shift-select to apply corner

Use an object selection method or hold down Shift and select an object to create a
sharp corner

If you select lines, arcs, or polylines, their lengths adjust to accommodate the fillet
arc. You can hold down Shift while selecting the objects to override the current fillet
radius with a value of 0.

If the selected objects are straight line segments of a 2D polyline, they can be adjacent
or separated by one other segment. If they are separated by another polyline segment,
FILLET deletes the segment that separates them and replaces it with the fillet.

More than one fillet can exist between arcs and circles. Select the objects close
to where you want the endpoints of the fillet.

FILLET does not trim circles; the fillet arc meets the circle smoothly.

If you select a 3D solid, you can select multiple edges, but you must select the edges
individually. (Not available in AutoCAD LT.)

Undo

Reverses the previous action in the command.

Polyline

Inserts fillet arcs at each vertex of a 2D polyline where two line segments meet.

Select 2D polyline

If one arc segment separates two line segments that converge as they approach the
arc segment, FILLET removes the arc segment and replaces it with a fillet arc.

Radius

Defines the radius of the fillet arc.

The value you enter becomes the current radius for subsequent FILLET commands. Changing
this value does not affect existing fillet arcs.

Trim

Controls whether FILLET trims the selected edges to the fillet arc endpoints.

Multiple

Rounds the edges of more than one set of objects.

Edge

(Not available in AutoCAD LT.)

Selects a single edge. You can continue to select single edges until you press Enter.

If you select three or more edges that converge at a vertex to form the corner of
a box, FILLET computes a vertex blend that is part of a sphere if the three incident
fillets have the same radii.

Chain

Changes from selection of single edges to selection of sequential tangent edges, called
a chain selection.

• Edge Chain. Selects a tangential sequence of edges when you select a single edge. For example,
  if you select an edge on the top of a 3D solid box, FILLET also selects the other
  tangential edges on the top.
  
• Edge. Switches to a single-edge selection mode.

Loop

Specifies a loop of edges on the face of a solid. For any edge, there are two possible
loops. After selecting a loop edge, you are prompted to Accept the current selection, or choose the Next loop.

Radius

Defines the radius of the fillet arc.