Generally the spectrum baseline is not perfect. One may thus want to subtract a baseline, calculated by a least-squares fit, excluding the intervals defined in command line (17) 3-6 and 8-11 (in velocity units according to (11)) in which the observed line is supposed to be present. This is done by command line (18), which computes a 4th degree polynomial (argument of the BASE command), plots the result superimposed on the preceding plot, then subtracts this polynomial from the spectrum. The original version of the spectrum is copied in the second "memory" of the system (T memory). The result of the baseline subtraction, in memory R, is displayed by the PLOT command (19). Note that the limits defined in (12,13) stay in use.