Mirror objects around an axis

Use Transform > Mirror by Reference Line to mirror objects around a defined axis using a reference line.

Use Mirror by Reference Line to mirror objects around a reference line.

To mirror objects around an axis...

Select the objects to mirror.
Select the Mirror by Reference Line icon or select Edit > Transform > Mirror by Reference Line.

Click to mark two reference points of the mirror axis.
Press Ctrl to constrain the angle of the axis to 15° increments.