Mirror sketch element on an axis or copy it with simultaneously pressed Ctrl key.
Mirror selection (and delete original):
Select the sketch element.
-> The element is marked in pink.
Click on the button Mirror selection .
Choose the mirror axis.
--> The element is mirrored. (The original is deleted.)
Mirror selection (and keep original):
Press Ctrl key and choose the mirror axis.
--> The element is mirrored. (The original is kept.)
