The infinite momentum frame (IMF) was originally introduced to provide a physical interpretationof the Bjorken variable x b j = Q 2 2 M ν ν measured in deep inelastic lepton -proton scattering ℓ ℓ p → → ℓ ℓ ′ ′ X in Feynman's parton model. (Here Q 2 = − − q 2 is the square of the spacelike momentum transfer imparted by the lepton and ν ν = E ℓ ℓ − − E ℓ ℓ ′ ′ is the energy transferred in the proton's rest frame.) If one considers a hypothetical Lorentz frame where the observer is moving at infinite momentum, P → → ∞ ∞ , in the negative z ^ ^ direction, then x b j can be interpreted as the longitudinal momentum fraction x = k z P z carried by the struck quark (or "parton") in the incoming fast moving proton. The structure function of the proton measured in the experiment is then given by the square of its instant-form wave function boosted to infinite momentum.