1. Enter the radius of the arc in the options tool bar.
2. Click the base entity close to the end where the arc should be connected.
3. Specify the end point of the arc.

QCAD Professional:

• Instead of an arc or a line, you can also choose an ellipse arc or spline as base entity.
• Entities inside blocks can be chosen as base entities as well.
• Tick the "Radius" check box in the options toolbar to fix the arc radius to the chosen value. Otherwise, the arc is constructed from the start point to the position of the mouse cursor.