These two meters sound very much alike to me and they can often be used interchangeably.

For example: it's a trivial thing to change the meter of "An der schönen blaue Donau" from 3/4 to 6/8. Redundant, yes, but the change is perfectly invertible.

Edit: So a compound meter is one that uses 1/8 notes in a way that cannot be changed into the simple 3/4 because the 1/8 note falls between the 1/4 notes in 3/4? Which means not all 6/8, 12/8 etc are truly compound.

Edit 2: On consideration, I no longer think the resolution (1/8 vs 1/4) is the problem. 6/8 can be interpreted as basically a 4/4 with implicit triplets while 3/4 has explicit triplets. Think of the percussion track of "Black Velvet" (12/8). This could be a slow 4/4, if you only hear the bass drum and the snare, but the bass plays triplets.

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    @EmanuelLandeholm then you are making a mistake. – Carl Witthoft Aug 28 at 12:45
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    @EmanuelLandeholm -- if you want to engage with any art form, you need to be able to embrace magic, i.e., the inexplicable. Craft lies in the part that can be analyzed; art lies in the part that is magic. The meter of a piece is chosen by a composer (or a transcriber) to communicate the feel of a piece. Of course feel is an ill-defined term, but music isn't accounting; if you want to understand music you have to spend some time with the natives to learn about their ways. The idea of feel may be ill-defined, but you can hear it in dynamics, articulation, phrasing, timbre, etc. – David Bowling Aug 28 at 14:15
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    Not entertaining answers referring to 'feel' means you won't allow yourself to accept this - and other aspects of music. – Tim Aug 28 at 15:47
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    The "feel" of a meter primarily refers to which subdivisions are accented. That aspect, at least, is not an ambiguous or "magic" concept. – Kyle Strand Aug 28 at 20:56
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    I don't have enough background to write up a full answer, but I think this is a very helpful visual + audio demo about the differences: megan-vo.github.io/basic-beats – Tyler W Aug 29 at 4:09

They are not very much alike, and cannot often (if ever!) be used interchangeably. So, the question itself is under false premises.

3/4 is 3 beats of one crotchet each. Counted 1&2&3& 1&2&3& etc.

6/8 is 6 beats of one quaver each. Counted as 123456 123456 etc.

3/4 therefore is simple - it could be 1 2 3 1 2 3.

6/8 is compound, as it's really two counts in one. It can (and often is) counted as 1--2-- 1--2--. So each trio of quavers counts as a beat. Think marching - l, r, l, r, or l-- r-- l-- r--. Or, as shown above.

True - they both contain the same value of notes, and that's usually where the confusion comes from. But that's where the similarity ends. It's probably one of the most difficult concept in time signatures to understand - and explain...

EDIT: answering the OP edit. 3/8 is similar to 3/4, but uses quavers. 6/8, 9/8, 12/8 are all (truly) compound times, as they can be split into 2, 3 and 4 respectively.

An important fact that was omitted here is that in 3/4, the first beat is emphasised (as in a lot of music), and the beats 2 and 3 are less so. Whereas in 6/8, beat 1 is most emphasised, but beat 4 ( or the second of 2 if we count two 'triplets') is also emphasised - but less so than beat 1. Rather like beats 1 and 3, in 4/4. Thank you, Monty Harder, for pointing this out.

  • Comments are not for extended discussion; this conversation has been moved to chat. – Dom Aug 28 at 13:09
  • Please use the chat above for discussion on this answer. Comments that are not asking for clarification or for improving the answer will be deleted. – Dom Aug 29 at 14:30

This very much depends on what tradition you're working in, what information you're trying to convey, and who will be reading it.

As many other answers have noted, the "feel" of the two meters is often thought to be different. This is because 6/8, in traditions that rely on sheet music written in the western style, is nearly universally interpreted as two groupings of three beats with strong emphasis on the first beat and a situational weak emphasis on the 4th. By contrast, 3/4 is interpreted as either one grouping of three beats, or as three groupings of one beat. Playing the 3 against the 2 is what makes 6/8 a compound meter, and when conducted by a human, the pulse is conveyed as 2 beats per measure.

When we start discussing waltzes the distinction becomes more or less muddled depending on your perspective. From a technical standpoint, you could take your copy of "An der schönen blaue Donau", scratch out all of the 3/4 markings, replace them with 6/8 and the strict definition of note-duration would still be accurate. The trouble is that waltzes are not played "straight" even when written in 3/4, and a major part of their tradition is to extend the duration of specific beats depending on the piece.

Very few musicians would be able to sight read your 6/8 waltz, and those who could would find it an annoying way to notate it. With a live orchestra, the conductor may also end up dropping the baton on beats that don't line up with the intended primary pulse of the piece, however this would get sorted after an initial sight-read and the realization that there should be 3 beats per measure instead of 2. A really wonderful rule of thumb for music engraving in general is to make the pulse visible to the musician, and you can see this in the beaming of 8th notes very clearly.

Notably, all of this goes out of the window if you're not working with acoustic instruments or a conducted orchestra. There are certain situations, in pieces of music that have multiple time signature changes and a compound pulse, where it may be appropriate to notate a measure in 6/8 when it has 3 pulses per bar. It ends up being completely contextual. If you're composing electronic music, your equipment may not support 6/8 and it will be easy enough to imply the same feel of stressed and unstressed beats in 3/4.

Writing music and notating music are not equivalent processes, and very often notation falls short of what the composer is trying to convey. When that happens, we must rely upon convention, and in this case the convention is that the two meters have different primary pulse divisions.

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    You said the same thing as the other people, but in a way that made a lot more sense. At least, when I was done reading it, I understood it in a way that I didn't even after reading two other answers. – ErikE Aug 28 at 18:55
  • @ErikE Thank you. I've spent time in the past playing in live ensembles in multiple genres/traditions, but I've spent the last 5 years doing nothing but avant-garde electronic composition. I have to say that provides some serious perspective on the limitations of sheet music! Beyond that, I've really had to learn how much of music education is learning what the composer is trying to convey, in addition to physically operating an instrument or learning theory. – Skoddie Aug 28 at 18:58

I should just rename myself "Mr.Dolmetsch Quoter"

in music, there are three kinds of meter:
simple in simple meter, each beat is normally subdivided into two parts, and the note receiving the beat is always a standard note value (i.e. a crotchet (quarter note), etc.)
in compound meter, each beat is normally subdivided into three parts, and the note receiving the beat is always a dotted note value (i.e. a dotted crotchet ( dotted quarter note), etc.) This is because a dotted note value may always be easily divided into three equal notes (i.e. a dotted crotchet (quarter note) = 3 quavers (eighth notes))
asymmetrical meters have an odd number of subdivisions, which means that the bar (measure) cannot be divided into equal beats. This type of meter is easy to recognize, since the top number is an odd number that is indivisible by 3 (i.e. 5, 7, 11, 13, etc.)

  • So if I straightforwardly transcribe "Donau" to 6/8 it magically becomes compound meter? – Emanuel Landeholm Aug 28 at 12:41
  • Surely anyone can quote - but a personal note is so much nicer. And this quote isn't the easiest to digest. It's turning into more than a can of worms. – Tim Aug 28 at 12:42
  • @Tim I'm not here to be nice, I'm here to learn music theory. – Emanuel Landeholm Aug 28 at 12:43
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    One of our main tenets is 'be nice'. What's your comment to do with anything? – Tim Aug 28 at 12:46
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    @EmanuelLandeholm please, please read through all that's been posted on this question,. I think you are entrenched in what you believe, and won't allow yourself to be swayed elsewhere, All that's stated is fact, and I hope, explained in an easy to understand format. It's what every experienced musician knows and understands, and is exactly as stated. Otherwise it'll bet downvoted - believe me! – Tim Aug 28 at 12:53

It's a convention that 6/8 is different from 3/4. Historically, time signatures represented tempo as well as note arrangement. A 3/4 time signature represents 3 beats in a measure using a quarter-note as a single beat. A 6/8 time signature represents 2 beats in a measure divided into triplets (else one could use 2/4). To hear the difference, try a waltz (Waltz Across Texas or Beautiful Blue Danube) and compare to a march (Washington Post or Semper Fidelis). The song America from West Side Story features alternating 3/4 and 6/8 measures (I don't know how this is notated; I think I read that each measure gets a fresh time signature. I would probably have used 6/8 with beaming and slurs.)

There are subtleties within each time signature also. The Star Spangled Banner isn't a waltz (nor is America AKA God Save The Queen). Jigs ar not marches.

  • Helpful, thanks. – Emanuel Landeholm Aug 29 at 4:43
  • Come to think of it, you totally could play "The star spangled banner" as a waltz though. :D – Emanuel Landeholm Aug 29 at 4:57
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    Star Spangled Banner as a waltz? Just checked 10 different versions, and 9 were 3/4. Waltzes are 3/4. The other (very spurious!) version was in 4/4 - couldn't believe it. But it would never be written or played (properly!) in anything bt 3/4. Never heard of 'America' (except from West Side Story - in 6/8 - but a red herring - but the U.K. God Save the Queen is also in 3/4, never 6/8! And - America (Bernstein) is usually written in 6/8. But he knew what he was doing! – Tim Aug 29 at 11:24

Almost all uses of the time signatures 3/8, 6/8, 9/8, etc. are due to a flaw in the notation for time signatures: there is no way to indicate that a dotted note has the beat.

If 3/4 were written 3/♩ then 6/8 would be written 2/♩. and they could both be interpreted in the same way: 3/♩ would mean that the measure is divided into 3 beats and each of those will probably be divided in twos most of the time (otherwise you would have written 3/♩.), and 2/♩. would mean that the measure is divided into 2 beats and each of those will probably be divided in threes most of the time (otherwise you would have written 2/♩).

Calling 6/8 a compound meter and 3/4 simple is an overcomplicated way of explaining around what is really a simple notational limitation.

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