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Uc Riverside: Twin Paradox
To make sense of this, you have to first see the Twin Paradox: Introduction, which is the special relativity version and is linked to this page. This general relativity explanation views the paradox from a gravitational point of view.
Oswego City School District
Regents Exam Prep Center: General "Transformation" Vocabulary
This site offers a short discussion of transformations. The discussion has few pictures and expresses the transformations in terms of relative coordinates rather than pictures.