  The principal pivot transform (PPT) of a matrix A partitioned relative to an
invertible leading principal submatrix is a matrix B such that
  A [x_1^T x_2^T]^T = [y_1^T y_2^T]^T
  if and only if
  B [y_1^T x_2^T]^T = [x_1^T y_2^T]^T,
  where all vectors are partitioned conformally to A. The purpose of this paper
is to survey the properties and manifestations of PPTs relative to arbitrary
principal submatrices, make some new observations, present and possibly
motivate further applications of PPTs in matrix theory. We pay special
attention to PPTs of matrices whose principal minors are positive.
