Subject: How do power plants keep the A/C frequency in phase with each other?

I am under the impression that the wests electrical power grid is many power 
plants feeding power into a centralized grid for distribution to anyplace on 
that grid. I want to know how they kept the alternating current from all the 
different sources in phase with each other. I have heard that today it is done 
with a master sync signal from satelites to each power generating station. Is 
that true and how did they used to sync them pre-satelites?

Re: How do power plants keep the A/C frequency in phase with each other?