A transverse piece which adds support to a structure.Three of these knees are used to help support the cross member and keep it in situ....
And combined with a passenger safety cell, surrounded by crumple zones, and structural cross members to absorb impact, it all helps to make the car a more enticing package.
Since any metal covering would block the antenna's signal reception, holes must be blanked in the roof and structural cross members re-routed.