The calibration of measurements of relative distances between extragalactic objects; also known as the cosmological distance ladder. It is difficult to measure the distances of most galaxies directly because individual calibration stars, such as Cepheid variables, cannot be seen at great distances. On the other hand, the relative distances of galaxies can be measured by means of correlations such as the Tully–Fisher or Faber–Jackson relations. These will show that one galaxy is, for example, twice as far away as another, but they cannot give exact distances unless the true distance to at least one galaxy is known. Once the distance scale is established, astronomers can determine the Hubble constant with accuracy. As of 2017, the best measurement of the Hubble constant from distance-ladder techniques was 73.24 ± 1.74 km/s/Mpc.