  The projection postulate is a description of the effect on a quantum system,
assumed to be in a pure state, of a measurement of an observable with a
discrete spectrum, in nonrelativistic quantum mechanics. It is often called
"von Neumann's projection postulate" or "the L\"uders rule". This paper is an
examination of the versions of this postulate due to Dirac, von Neumann and
L\"uders. It is shown that Dirac, in 1930, proposed what is now generally known
as the projection postulate. Von Neumann, in 1932, gave a different theory
which only applies in special and rather unusual cases. L\"uders, in 1951,
rejected this theory and presented one which is the same as Dirac's. Treatments
of observables with continuous spectra by both Dirac and von Neumann are
criticised, and the possibility of a generalised version of the projection
postulate for this case is considered. The paper concludes with a discussion of
the status of the projection postulate (in its various forms) as a separate
postulate (independent of the other postulates of quantum mechanics) and as a
separate form of time development (in addition to the time-dependent
Schr\"odinger equation).
