Selection prerequisites: Any number of objects including at least one straight object
To mark a mirror:
•Double-click a straight object with the Selection Arrow tool.
If you haven’t yet marked a mirror when you choose Reflect, Sketchpad will automatically choose a straight object and mark it for you.
The marked straight object will flash briefly to indicate it has been marked as the mirror for future reflections. Once marked, this mirror will be used for all future reflections until you mark a different mirror.