If I have a sequence of 100 monotonically increasing numbers, and if I were to expand it back-to-back to 200 numbers. Is it the right way to understand "back-to-back" here, that it means the second half of the expanded 200 numbers will mirror the first half of the sequence, and will be monotonically decreasing?

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    I’m voting to close this question because this is a question about the mechanics of encoding, not the use of English. It could be answered on another site devoted to signal processing or by reference to the specification. To answer it on a technical level also requires a reference to which signal encoding standard is in question. Commented Nov 26, 2023 at 11:23
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    I've edited away the technical part and leave the question to be about the interpretation of "back-to-back". Hope that can be on-topic @PeterJennings
    – DannyNiu
    Commented Nov 27, 2023 at 0:52
  • The expression "back-to-back" is not normally used in this context and its meaning is not clear. My first thought was that you would be prepending numbers with negative index values.
    – Peter
    Commented Nov 27, 2023 at 0:56
  • @Peter The original question (before editing) asked the phrase as used in a signal processing standard. If we conclude that "back-to-back" is too ambiguous to be appropriate for use in technical specifications, then by all mean, do close this question.
    – DannyNiu
    Commented Nov 27, 2023 at 1:20
  • I don't think the question should be closed. It's true "back-to-back" doesn't have the meaning DannyNiu might have liked it to have, but that's no reason to close it. How about coming up with suggested alternatives that do have the sought meaning? Commented Nov 27, 2023 at 1:36

1 Answer 1


"Back to back" means that two things are right next to each other. It does not mean that one is a mirror image of the other. I see how you might guess that by looking at the words, but no, that's not what it means. If someone says, for example, "I took three classes back to back", he means that there was no gap between the classes. Like Math class goes from noon to 1:00, and then History is from 1 to 2 and Science is from 2 to 3. He has to rush from one class to another, there is no gap in between. In practice there could be a small gap, like Math goes from noon to 12:50 and then History is from 1:00 to 1:50, etc. But if Mathis from noon to 12:50 and his next class is History at 4:00, that's not "back to back". In your example, the writer means that the second group of numbers immediately follows the first group. I'd have to read the larger context to know exactly what he means. He may mean that the first group goes from 1 to 100 and then the second group continues with 101. That is, no gap between the numbers. Or he may mean that as the numbers are written there is no space on the page, or as they are transmitted electronically there is no time gap, etc.

  • The context was removed from the question to make it on-topic. But since you're a high-rep member on SO (I checked your profile), it probably make sense sharing some technical detail with you. Section Windowing.
    – DannyNiu
    Commented Nov 27, 2023 at 3:14
  • @DannyNiu - could be that your original interpretation is correct; the values in the Transform Window Sequence table look like one half of some bell-shaped curve when plotted (perhaps, but not necessarily, a Gaussian), so it makes sense that you'd flip the values to complete it. Commented Nov 28, 2023 at 1:05
  • @FilipMilovanović Very true. And as an ETSI spec, it uses comma instead of period as decimal points throughout the spec. I really hope there's an SE site dedicated to interpreting EU's dialect of English.
    – DannyNiu
    Commented Nov 28, 2023 at 3:29
  • The phrase "back to back" COULD be used for two things that are mirror images of each other. Maybe that's the case in whatever the context is that the OP is specifically asking about. But the phrase "back to back" does NOT mean that the two things are mirror images. It's like, I could say, "There are two apples in the basket", but that would be an example of "in the basket". You could not therefore conclude that any time I use the word basket I must be referring to apples. If the two are mirror images IN THIS EXAMPLE, fine, but that has nothing to do with the phrase "back to back".
    – Jay
    Commented Nov 30, 2023 at 0:15

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