Distribution of the frequency drift of oscillators
Start with your requirement for ppm error frequency and desired operating temperature range then define or choose the resonator that meets this criteria.  The most common AT cut quartz crystal comes with different angle cuts that affect the 3rd order temp vs ppm error curve and can be chosen over a very wide temp range or a narrow range.

I once computed the 3rd order coefficients to make a TCXO for under $1 using testers to bin XTAL curves in 10 seconds and varicaps in 1 second to match with a thermistor to computed DAC Vf to compensate for 3rd order tempco over -40 to +70'C within 1ppm . Now you can buy the 2ppm TCXO oscillators for a few bucks or less.

Room temp watch crystals use a parabolic tempco crystal centred at 28'C


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