#Corner fillet and chamfer

Click the corner of two lines. The size is set in the top bar.

#What happens

A fillet inserts an arc between the lines and adds two tangencies, one to each side. A chamfer inserts a segment at the required angle. Both stay constrained: move a side and the corner rebuilds itself.

#Why not draw the arc by hand

You can, but then both tangencies are yours to place, and either is easy to forget. A forgotten tangency is invisible in the picture — it shows up later, when the outline comes apart at the joint on the first change of a dimension.

#A hint

Corners are usually rounded last, once the outline is defined: before that they get in the way of snapping to corners and confuse the count of degrees of freedom.