Step 05 - Connecting the seat and back support.

127) Now lets connect the back support to the seat. In the Left Viewport, create a Line like this:

128) In Front Viewport move it to one of the sides.

129) And than chamfer the vertex located in the corner.

130) In any viewport, go to Spline subobject, select the spline, hold down shift key and move this line to the other side creating a new copy of it.

131) Go to Back viewport and click at Snap Toggle to enable snaping. Right click on the same icon and just enable Vertex Snaping.

132) Now create a Arc Shape from one vertex to other (of the support).

133) Attach this new Arc to the other two Splines.