  These lecture notes are a personal introduction to signed graphs,
concentrating on the aspects that have been most persistently interesting to
me. They are just a few corners of signed graph theory; I am leaving out a
great deal. The emphasis is on the way signed graphs arise naturally from
geometry, especially from the geometry of the classical root systems. Most of
the properties I discuss generalize those of unsigned graphs, but the
constructions and proofs are often more complicated. My aim is a coherent
presentation of the subject, with a few illustrative proofs and adequate
references. Hence the arrangement of the notes is topical with only occasional
remarks about the historical course of development. Though this is mainly an
expository survey, some of the results have not hitherto been published.
