  The `Salpeter Equation' which has long been known as the 3D version of the 4D
Bethe-Salpeter Equation under the Instantaneous Approximation, has a
well-defined rationale that stems from the half-century old Markov-Yukawa
Transversality Principle (MYTP) which not only effects an exact 3D reduction
from the original (4D) BS form, but also provides an equally exact
reconstruction of the 4D BS amplitude in terms of 3D ingredients. The second
aspect which is new, opens up a vista of applications to transition amplitudes
as 4D loop integrals.
  PACS: 03.65.-w ; 03.65.Co ; 11.10.Qr ; 11.10.St Keywords: Salpeter eq;
Markov-Yukawa Transversality Principle (MYTP); Covariant Instantaneity; 3D-4D
interlinkage; Vertex fn; 4D loop integrals
