The HERMES collaboration has measured double spin asymmetries using polarized deep inelastic electron scattering. Inclusive and semi-inclusive asymmetries were measured from proton and deuteron targets with high precision. Asymmetries of pions from the proton and of pions and kaons from the deuteron were measured for the first time. Polarized parton densities of the u, (u-bar), d, (d-bar), (s + (s-bar)) flavours were extracted from the inclusive and semi-inclusive spin asymmetries in a LO analysis. The densities of the up (down) quarks were determined with good precision to be positive (negative). The polarized density of the sea quarks is consistent with zero.
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