quasars are those quasars lacking powerful jets, with relatively weaker radio emission than the radio-loud population. The majority of quasars (about 90%) are radio-quiet. Broad absorption-line (BAL) quasars are quasars whose spectra exhibit broad absorption lines that are blueshifted relative to the quasar's rest frame, resulting from gas flowing outward from the active nucleus in the direction toward the observer. Broad absorption lines are found in about 10% of quasars, and BAL quasars are usually radio-quiet. In the rest-frame ultraviolet spectra of BAL quasars, broad absorption lines can be detected from ionized carbon, magnesium, silicon, nitrogen, and other elements. Type 2 (or Type II) quasars are quasars in which the accretion disc and broad emission lines are highly obscured by dense gas and dust. They are higher-luminosity counterparts of Type 2 Seyfert galaxies. Red quasars are quasars with optical colors that are redder than normal quasars, thought to be the result of moderate levels of dust extinction within the quasar host galaxy. Infrared surveys have demonstrated that red quasars make up a substantial fraction of the total quasar population. Optically violent variable (OVV) quasars are radio-loud quasars in which the jet is directed toward the observer. Relativistic beaming of the jet emission results in strong and rapid variability of the quasar brightness. OVV quasars are also considered to be a type of blazar. Weak emission line quasars are quasars having unusually faint emission lines in the ultraviolet/visible spectrum. Role in celestial reference systems Because quasars are extremely distant, bright, and small in apparent size, they are useful reference points in establishing a measurement grid on the sky. The International Celestial Reference System (ICRS) is based on hundreds of extra-galactic radio sources, mostly quasars, distributed around the entire sky. Because they are so distant, they are apparently stationary to current technology, yet their positions can be measured with the utmost accuracy by very-long-baseline interferometry (VLBI). The positions of most are known to 0.001 arcsecond or better, which is orders of magnitude more precise than the best optical measurements. Multiple quasars A grouping of two or more quasars on the sky can result from a chance alignment, where the quasars are not physically associated, from actual physical proximity, or from the effects of gravity bending the light of a single quasar into two or more images by gravitational lensing. When two quasars appear to be very close to each other as seen