Parallel (curve)

A parallel of a curve is the envelope of a family of congruent circles centered on the curve. It generalises the concept of parallel lines. It is sometimes called an offset but that often refers also to translation.

Alternatively, one can fix a circle and a point on the curve and take the envelope of the translations taking that point to the circle.

A curve that is a parallel of itself is autoparallel. The involute of a circle is an example.