Gas along the lines of sight to QSOs produces narrow absorption lines from ions of the most abundant elements: H, He, C, N, O, Si, S, Mg and Fe. Most lines have rest wavelengths of a few hundred Å but some have wavelengths λ ≥ 900 Å, sufficient that they can be redshifted into the optical. The absorbing gas is normally transparent, except for Lyman continuum absorption, and dust extinction at the highest columns, so we can see hundreds of gas clouds along a line of sight. We see more absorption from higher redshifts, because the universe was denser then and the absorbing “clouds” have roughly constant proper size.