you must have a "known" in order to measure an "unknown".... you can compare two "unknowns", but it's only a relative comparison. so you have "unknown-1" and "unknown-2", and you can compare them to find that "unknown-2" is twice the frequency of "unknown-1", but to actually KNOW the frequency of one or the other, you need to compare them with an objective standard. with your "unknowns" you can even have extremely high precision (i.e. repeatability), but not accuracy. now if you add a 10Mhz time base, and compare the unknown with it, now you know what the frequency is....