Example of appearing BAL troughs. This quasar has been observed in SDSS-DR7 (red curve) and in SDSS-DR9 (black curve). The two spectra have been scaled to have a surface unity between 5600 and 6200 Å. The normalized flux in the C iv region expressed in velocity is displayed in the inset. This quasar had was not detected as BAL in SDSS-DR7 (i.e. BI = 0) while it has BI = 1826  km s^-1 in the SDSS-DR9 spectrum.