I'm not in the travel industry but my understanding is that a legal connection is generally the shortest amount of time it would take to make a "normal" on-line connection between flights (i.e. Airline X to Airline X) at any given airport. This time allows you to get from gate to gate, and also allows your luggage to make the connection (hopefully!)
Most airlines have a general rule for legal domestic connections (usually :30) and legal international connections, though at some airports this may adjusted for local conditions, for example, with Northwest,
domestic is 30 minutes
domestic is 25 minutes
, Pinnacle or Mesaba to NW
is 35 minutes
to the US, though, has a 50 minute minimum connection.
Most reservation systems won't allow you to book (or will at least make it difficult to book an "illegal" connection -- and if you have a shorter-than-legal connection and miss it the airline might not be liable for the hotel/meals/your luggage, etc.
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