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I am using the "A Dictionary of Basic Japanese Grammar" to help me with Japanese studies. As explained from page 128 on I understood the difference between ごとに and おきに.

For example:

  • 2日おきに = every third day
  • 2日ごとに = every second day

The Answer what is the difference between ごとに and おきに? also backs this understanding.

Now in the mentioned grammar book there is one last example where both forms do yield the same meaning.

Quoting:

When a time expression precedes oki no or goto ni, there is no difference in meaning, if an event takes place at one point in time:
[電車]{でんしゃ}は[五分]{ごふん}おきに/ごとに[出]{で}る = The train leaves every five minutes.

I don't get the difference between this "time expression" and the other ones like 二日. Can somebody give me a hint here?

  • I don't think that anything special with Japanese. Time as measured in minutes/seconds/... is continous. If measured as an amount of days, it's discrete. Let A and B be two points in time, then there are days between A (eg Monday) and B (eg Thursday) and there are 5 minutes between A (eg 1:00am) and B (eg 1:05am) use the same expression in English as well. – blutorange Nov 21 '14 at 16:36
  • @blutorange You can leave that as an answer, if you like. – snailcar Nov 21 '14 at 17:37
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    I must have read a paper on this topic years ago. In short, the difference pretty much depends on how you perceive the noun. If you view it as a mass noun, then ごとに and おきに are the same. If you view it as a countable noun, the case becomes more complicated, depending on whether two types of things or a single type is involved. For example, オリンピック and 年 are two types of things, so there are 4年 between two オリンピック. But we often think days are time slots, which can be assigned to something. Therefore 2日おきに is more like 1日 between 2日, which only involves only a single type of noun. – Yang Muye Nov 21 '14 at 21:47
  • An even more impressive example than the Olympic Games is imo 4年おきに閏年を入れる. – blutorange Nov 21 '14 at 22:59
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毎【ごと】に means "every", so 2日ごとに is "every second day".

On the other hand, X置【お】きに literally means "leaving (an amount of time/space/...) X (between each occurence)". It comes from the verb 置く, "to put", "to place", "to leave (sth. somewhere)".

Here is an article from NHK's 身近なことばの疑問にお答えします about ごとに and おきに.

So how come おきに sometimes means the same as ごとに, and sometimes not? Let's think about English for a moment, the same phenomenon happens in English as well.

Let A and B be two events separated by a certain amount of time. How much time is there between A and B? You might be tempted to answer B minus A, but there are two different answers, depending on how we count time:

(a) There are 5 minutes between A = 1:05 am and B = 1:10 am. B - A = 5 minutes. This is 5分おきに - leaving 5 minutes between A and B.

(b) There are 2 days between A = Monday and B = Thursday. B - A = 3 days, not(!) two. This is 2日おきに - leaving two days between A and B.

The difference between these two cases is that in (a), time as measured in hours, minutes, seconds etc. is considered continous (uncountable) - there's 5 minutes, 5.3 minutes, 5.000321 minutes and so on. In scenario (b), time counted as weekdays or as a number of days is considered discrete (countable) - there's Monday and Tuesday, but nothing in between, we don't talk about Monday and a half.

To illustrate this point:

1 2 3 4 5

How many numbers are there between 1 and 5? Three, namely 2, 3, and 4.

The scale of a measuring cup (for water)

|---+---+---+---+---| 
0   1   2   3   4   5 (deciliter)

How many milliliters are there between 1 dl and 5 dl? The answer is 400 ml = 4 dl, not 3 dl.


Conclusion:

Both おきに and ごとに have got only one basic meaning. Depending on the noun they apply to, both can refer to the same (temporal) interval.

  • 2日ごとに every 2 (week) days
  • 2分ごとに every 2 minutes
  • 2日おきに 2 (week) days between each occurence
  • 2分おきに 2 minutes between each occurence

A day consists of 24 hours. Note the difference between 48 hours between two events and 2 days between two events. And would you say that there are 2880 minutes between Monday and Thursday?

If you were looking at a clock face with each minute marked individually, 2分おきに might mean something different.

Note that this is not limited to temporal intervals. 一行おきに書く "write on every other line", 5メートルおきに杭【くい】を立てる "place stakes with a space of five meters in between".


To put it another way, you take the open interval (A,B). In the case of a continous variable, everything just a split second after 1:00 am is part of that interval, and thus you get 5 minutes between 1:00 am and 1:05 am. But in the case of a discrete variable, you get less. Consider (Monday,Thursday), Monday midnight + 1hour is not part of the interval because you're counting only in days, but not hours. There's only Monday and Tuesday, but nothing in between. So you get only two days inside the interval (Monday,Thursday), even though Thursday is 3 days after Monday.

  • For the record, my (non-linguistic) explanation for the case Yang Muye mentioned would be the different length of the two types of things involved, Olympic Games and years. The Olympic Games take place over the course of a few weeks, much shorter than a year, so instead of year-time-slots, we think about the continous time (mass noun) between two Olympic Games. Which happens to be approx. 4.0 (not 4) years, or ~208 weeks. Thus you can say 「オリンピックは4年おきに開催される」. – blutorange Nov 21 '14 at 22:21
  • I forget to mention that when X is viewed as slots, Xごとに is actually every X on average. – Yang Muye Nov 21 '14 at 22:48

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