I 've always thought the mains frequency, either 50 or 60Hz in most parts of the world, as one of those things that don't really change.
Today I was testing the software for an oscilloscope and more precisely a light-sensitive trigger, which is supposed to trigger a signal following the frequency of a flashing light. As I tested with an office lamp this frequency was naturally 50Hz in my case.
Inputting a 50Hz sine wave into the oscilloscope I got a "almost" steady waveform, in a sense that the triggering rate was "exactly" the same as the waveform frequency. In reality to really get it stable I had to tune the input frequency continuously, though only slightly. In a few minutes I went around the range 50.001-50.003Hz.
A quick Google suggested typical values for "mains frequency tolerance" is +-1% i.e. about 0.5Hz. In my case it looks like they did a good job.
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