Inside fillet
The Inside Fillet 
tool is used to
create an inside fillet on a corner of a polyline by specifying the fillet
radius.
Creating an inside fillet
Type in the fillet radius and click near a corner to create an inside fillet.
| The side of the segment you click determines whether the inside fillet will have a major arc or a minor arc. |
Creating inside fillets on all corners
You can apply inside fillet to all corners by holding down the Ctrl key when clicking near a corner.
| The inside fillet is created only if both the line segments meeting at the corner are straight line segments, and of sufficient length. |