  We propose a general construction of quantum states for linear canonical
quantum fields on a manifold, which encompasses and generalizes the "standard"
procedures existing in textbooks. Our method provides pure and mixed states on
the same footing. A large class of examples finds a simple and unified
treatment in our approach. Applications discussed here include thermodynamical
equilibrium states for Minkowski fields and quantum field theory in the
Rindler's and in the open de Sitter universes. Our approach puts the above
examples into perspective and unravels new possibilities for quantization. We
call our generalization "extended canonical quantization" as it is suited to
attack cases not directly covered by the standard canonical approach.
