We calculate and identify the counterparts of zero-norm states in the old covariant first quantised (OCFQ) spectrum of open bosonic string in two other quantization schemes of string theory, namely the light-cone DDF zero-norm states and the off-shell BRST zero-norm states (with ghost) in the Witten string field theory (WSFT). In particular, special attention is paid to the inter-particle zero-norm states in all quantization schemes. For the case of the off-shell BRST zero-norm states, we impose the no ghost conditions and recover exactly two types of on-shell zero-norm states in the OCFQ string spectrum for the first few low-lying mass levels. We then show that off-shell gauge transformations of WSFT are identical to the on-shell stringy gauge symmetries generated by two types of zero-norm states in the generalized massive σ-model approach of string theory. The high energy limit of these stringy gauge symmetries was recently used to calculate the proportionality constants, conjectured by Gross, among high energy scattering amplitudes of different string states.Based on these zero-norm state calculations, we have thus related gauge symmetry of WSFT to the high-energy stringy symmetry of Gross.Recently it was discovered that [1, 2] the high-energy limit α ′ → ∞ of stringy Ward identities, derived from the decoupling of two types of zero-norm states in the OCFQ spectrum, imply an infinite number of linear relations among high energy scattering amplitudes of different string states with the same momenta. Moreover, these linear relations can be used to fix the proportionality constants between high energy scattering amplitudes of different string states algebraically at each fixed mass level. Thus there is only one independent component of high energy scattering amplitude at each fixed mass level. For the case of string-tree amplitudes, a general formula can even be given to express all high-energy stringy scattering amplitudes for arbitrary mass levels in terms of those of tachyons [1,3].These zero-norm state calculations are independent of the high-energy saddle-point calculations of Gross and Mende [4], Gross [5] and Gross and Manes [6]. In fact, the results of saddle-point calculations by those authors were found [1-3] to be inconsistent with high energy stringy Ward identities of zero-norm state calculations, and thus could threat the validity of unitarity of string perturbation theory. A corrected saddle-point calculation was given in [3], where the missing terms of the calculations in Refs [4,5,6] were identified to recover the stringy Ward identities.The importance of zero-norm states and their implication on stringy symmetries were first pointed out in the context of massive σ-model approach of string theory [7]. Some implications of the corresponding stringy Ward identities on the scattering amplitudes were discussed in [8]. On the other hand, zero-norm states were also shown [9] to carry the spacetime ω ∞ symmetry [10] charges of 2D string theory [11]. This is in parallel with the work of [12] where the gr...