Per definition, ‘
married segment’ logic allows participating carriers to control their flight inventory. When air segments are booked, a marriage can be established by the airline between two or more segments, requiring them to be processed as a unit.
Individual segments within a marriage cannot be priced or ticketed separately.
Two flights – one unit
The term ‘married segment’ is used by airlines to view inventory of two or more segments of an origin and destination as one instead of individual legs. For instance, Miami (MIA) to Los Angeles (LAX) can be a direct flight (non-stop), or one can e.g. connect in Houston (IAH). For the latter, these are two segments: MIA-IAH and IAH-LAX.
Now, these two segments can be married or not. If they are married, it means that unlike no-fault divorce, it is extremely rare to successfully break those ‘married segments’ and for example add a day in Houston to visit the space center.
The reason is that the middle city (in this case Houston) is most likely a hub of one airline (in this case United), which means that this airline can ‘control’ the market for flights with a destination (or stopover) of this city (in this example Houston). While in this example United competes with American on a flight from MIA to LAX, and hence need to provide competitive fares for travelers who take the burden of a connecting flight (meaning getting out of an airplane and catching another one).
In many cases travelers who just want to fly from Miami to Houston may end up paying more to United than people flying through Houston to Los Angeles. Smart people (or agents) obviously came up with booking MIA-LAX at the cheaper rate and then breaking apart the second leg (IAH-LAX), probably even abandoning that leg; hence airlines needed to come up with a concept to allow such a break-up only with huge penalties or do not allow such a break-up at all. This led to the term ‘married segments’.
This applies to any connections to hub airports: MIA-FRA-MUC may be cheaper than MIA-FRA on Lufthansa, MIA-ATL to anywhere may be cheaper than MIA-ATL on Delta and so on.
https://www.travel-industry-blog.com/travel-technology/married-segment/