Routes - IFR Enroute Low vs IFR Enroute High

FlightAware ADS-B Flight Tracking
1

I like watching the flight trails laid against the IFR enroute charts. I’m a non-aviator, so hoping someone can educate me as to when aircraft should generally transition from the low routes to the high routes and vice-versa. Is that pretty much determined by altitude?

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High altitude (Jet Routes or Q routes) start at FL180 (or 18,000’ pressure altitude).

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Awesome, thanks.

19-20.