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Texts on relativity, either special or general, talk conspicuously about rates of clocks but I quite don't understand this: does relativity theory assign some absolute meaning to the rate of a clock, or is it that - explicitly or implicitly - we are always comparing the rates of two clocks? As an example, this quote from Einstein's popular exposition Relativity: The Special and General Theory:
"in every gravitational field, a clock will go more quickly or less quickly, according to the position in which the clock is situated (at rest)"
"Thus the clock goes more slowly if set up in the neighbourhood of ponderable masses." (A. Einstein, The Foundation of the General Theory of Relativity, §22)