  This expository article introduces the topic of roots in a compact Lie group.
Compared to the many other treatments of this standard topic, I intended for
mine to be relatively elementary, example-driven, and free of unnecessary
abstractions. Some familiarity with matrix groups and with maximal tori is
assumed.
  This article is self-contained, but is also intended to serve as a
supplemental 10th chapter of an eventual new edition of my textbook, ``Matrix
Groups for Undergraduates.''
