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Also remember that at any time, one end of the clubshaft is pointing to the plane line (which continues to infinity in each direction) or that the clubshaft is parallel to the plane line.

Correction: If one end of the clubshaft is pointing at the plane line, it doesn't guarantee that the clubshaft is on plane. For example the clubshaft could be under-plane or over-plane and be pointing at the plane line, and obviously not "on plane". (This after reading an article on GolfBetterProductions.com)