The hadron spectrum is characterized by the valence quark degrees of
freedom, even though scattering (DIS) data shows that hadrons also have sea quarks and gluon constituents.
The Dirac equation demonstrates that these two features can coexist for relativistic dynamics. Dirac bound
states of an electron have an unlimited number of electron-positron pair constituents, while the spectrum is
determined by a single electron equation. Confined fermion-antifermion states can be realized in gauge theory by
imposing a non-vanishing boundary condition on Gauss' law. Only the simplest homogeneous solution,
$A^0(\boldsymbol{x}) \propto \boldsymbol{x}$, is compatible with translation invariance, and then only for neutral
states. This results in a linear instantaneous potential, similar to the ${\mathcal O}(\alpha_s^0)$ potential of the
quark model. The bound states are described by equal-time wave functions in all frames, are rotationally invariant
in the rest frame and have a dynamically realized boost covariance. Their electromagnetic form factors are gauge
invariant and their parton distributions have contributions from sea quarks at low $x_{Bj}$. The boost from the
rest frame to the infinite momentum frame reveals an interesting difference wrt. wave functions defined at
equal light-cone time. The states thus constructed are candidates for ${\mathcal O}(\alpha_s^0)$ asymptotic
($in$ and $out$) states of the QCD $S$-matrix.
This approach is described in the lecture notes of arXiv:1402.5005.