Line-to-line, line-to-plane, and plane-to-plane parallelism can step up and down: proving parallelism in space often means shuttling among all three.
Module goal. Master systematically the tests and properties of parallel lines, line-and-plane, and plane-and-plane, and learn to convert freely among them.
eastmath.com · 28.6 Parallelism in Space: Tests and Properties · 28.6.5 Converting Between Kinds of Parallelism