By the way

We use this expression to introduce a different topic or new idea which may be only partly related to what we were talking about.

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Written	Yes
Spoken	Yes
Formal	Yes
Informal	Yes

Example 1

Is it Alicia's birthday next week?
Yes, it is. By the way, she's split up with her boyfriend, so don't say anything about him.

Example 2

See you on Saturday evening at 7.00, then.
Right. Oh, by the way, where's the best place to park?

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