Airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are: indicated airspeed ("IAS"), calibrated airspeed ("CAS"), true airspeed ("TAS"), equivalent airspeed ("EAS") and density airspeed. During cruising flight at altitudes, airspeeds, and temperatures common for airliners, the four speeds trace a shape that looks like the mathematical square root symbol (v). Starting with indicated airspeed, calibrated is normally very close to the indicated airspeed, while equivalent is normally less than both indicated and calibrated, and true is normally higher than the other three.