When is it that you can't reserve a specific seat on a plane beforehand?

Two of my friends flew Scandinavian Airlines from Norway to Denmark, and from Denmark via Dubai to India with Emirates.

They thought they could select seats 24 hrs before departure, but when they logged on it said that because of the first flight with SAS, it wasn't possible.

So, my question is: how can you know whether you are able to select your seat? Does it apply to all airlines that you can't do it if you have had a connecting flight with another airline? How about if you want to select a seat on the first airline, but have a connecting flight afterwards?

Specifically, I'm thinking about Norway to Heathrow with British Airways, and Cathay Pacific to Hong Kong. (The last leg is most important)

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When is it that you can't reserve a specific seat on a plane beforehand? - Airplane Window Opened

How long before flight can you select seats?

Unless you have booked a Basic ticket (economy, hand baggage only), you can choose your seat for free when check-in opens (24 hours before departure).

Can you select seats before check-in?

If you think that you can choose whichever seats that are not chosen prior to airport check-in, then you're wrong! This is simply because airlines reserve some seats for emergency or operational purposes. It's true that sometimes airlines will have already reserved some seats prior to online check-in.

Can you choose a seat after booking a flight?

When should I select my airline seat? After you finish booking your flight (or during the process, depending on the airline), you should click to view the seat map and make your pick. If you book your flight with friends or family members on the same reservation, they will likely be assigned seats nearby automatically.

Can I see what seats are available on a flight before booking?

Depending on your airline, you're almost certainly able to get a peak at the seat map during the check-in process, which starts 24 hours before departure. This usually comes with the chance to select or purchase seats, and as such will show you how many open seats are available the day before your flight.

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Images: Rui Chaves, Tim Gouw, Daniel Frese, Anugrah Lohiya