The null-plane quantization is studied with the emphasis on the P+ = 0 mode, by using Dirac's quantization for constrained systems. This mode is eliminated from the Hilbert space and the physical vacuum can be defined in a kinematical way. It enables us to construct the physical Fock space kinematically. Poincaré invariance is also studied in detail.
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Maskawa et al. (1976) studied this question.