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At the first glance, this phrase to me sounds like airfare for one-way. But I was told by a native English speaker that it meant airfare for round-trip. So I guess there should be no such a confusion with a native English speaker. Nevertheless, I am wondering, why did people not adopt anything like round-trip airfare or round-way airfare in the first place? Thanks for any comments about this.
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In the early days of railways many people were migrant labourers in the UK. Migrant workers would buy a one-way journey, or single-journey ticket, a 'single'. The ticket-office clerks got into the habit of asking people if they wanted to return to the same railway station. "Do you want to return." might have become "Do you want a return ticket?". By leaving out the word 'ticket' as an obvious part of a question in a ticket office, this would have become "Do you want a return?", then "Single or return?"

'Single' and 'return' both apply to a journey, as is obvious to a ticket clerk. The rule of grammar which allows obvious words to be omitted is called ellipsis.
Wow, I did not expect such a detailed explanation! It's great. Thank you very much, Patrick. BTW, welcome to these forums!
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Osee, I think there is regional variation in this.

In your post, it read to me as though you were paying for her flight out. Thus, at some point in the future, she would have to pay to return, meaning "only that part of her trip in which she returns home."

In the US, we refer to "one-way" or "round-trip." 

Elsewhere, if you say "your return fare" it could mean what we call a "round trip."
Hi GG, no, I am NOT going to pay her flights either coming to Canada or being back to China; she will pay all flights. So, considering this, did you suggest that, in US, using return airfare is not good to refer to airfare for "coming and being back" or "round-trip/way?"

If so, maybe in future I should always stick to "round-trip airfare;" it might be weird to people in some other countries, but it gets things straight for all people, right?

BTW, I notice you said one-way and round-trip, why there is no "round-way?" Thanks.
Grammar GeekOsee, I think there is regional variation in this.

In your post, it read to me as though you were paying for her flight out. Thus, at some point in the future, she would have to pay to return, meaning "only that part of her trip in which she returns home."

In the US, we refer to "one-way" or "round-trip."

Elsewhere, if you say "your return fare" it could mean what we call a "round trip."

Osee
BTW, I notice you said one-way and round-trip, why there is no "round-way?" Thanks.

An interesting question. I don't have an interesting answer!

I hope that "roundtrip" would be clear to most people, but I don't know. There also used to be a fare type that I don't see anymore, which was a round-trip with an open-return. Because it used to be that a round-trip ticket was much, much less expensive than two one-way tickets, you could book a round trip without specifying a return day. I don't know if airlines still do that.
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Grammar Geek
Osee
BTW, I notice you said one-way and round-trip, why there is no "round-way?" Thanks.

An interesting question. I don't have an interesting answer!

I hope that "roundtrip" would be clear to most people, but I don't know. There also used to be a fare type that I don't see anymore, which was a round-trip with an open-return. Because it used to be that a round-trip ticket was much, much less expensive than two one-way tickets, you could book a round trip without specifying a return day. I don't know if airlines still do that.

As far as I know, you have to specifying a return day, but you can pay like $100 to change it to a date within one year of the "starting date of the round-trip." Is there a name for this blue part?

Taken together with 'the', making: 'the starting date of the round trip', I would call that a noun phrase.
Thanks Patrick! That was very helpful..
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