Frequency coverage of redshifted fine structure lines for some current (and future) key telescopes as a function of redshift. The following lines are plotted (in order of increasing frequency, given in GHz]): CIν492.2, CO(6–5)ν691.5, CIν809.3, [N ii]ν1461.1 (=[N ii] _205 μm), [C ii]ν1900.5, [O i]ν2060, [N ii]ν2460, [O iii]ν3393, [O i]ν4745, [O iii]ν5786). The left panel shows the coverage for the PdBI and CARMA, the middle panel the respective coverages for the EVLA and the IRAM 30 m telescope and the right panel for ALMA (bands 2–9; band 5 is not fully funded and thus appears in lighter color). The three black points in the left and middle panels highlight the positions of the transitions in J1148+5251 discussed in this Letter (CO(6–5), [N ii] _205 μm and [C ii]).