https://thepointsguy.com/news/southwest-airlines-icelandair-partnership-how-to-book/
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Southwest Airlines’ brand-new partnership with Icelandair is officially live and available for booking — but be warned, it’ll be a bit limited for the time being.
Beginning Thursday, customers are able to book itineraries featuring connections between Southwest and Icelandair via Southwest’s robust station at Baltimore/Washington International Thurgood Marshall Airport (BWI).
That means you could start a trip with a flight on a Southwest jet and connect seamlessly — both you and your bags — to an Icelandair flight to Keflavik Airport (KEF) in Reykjavik. From there, you could even continue onward, on the same itinerary, to additional destinations in Europe.