Why is 3/4 a simple meter while 6/8 is a compound meter?

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.)
compound
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
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.)

Answer 4

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.

Answer 5

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.

Answer 6

An old question, but the existing answer don't seem to get directly to the point.

The matter is entirely about how the beat is subdivided.

• Simple meters subdivide beats by two.

• Compound meters divide beats by three.

When the top number is multiple of 3 (greater than 3 itself) so 6, 9, 12, etc. then the meter is compound not simple. Divide that top number by 3 to get the number of beats per measure. Ex. 12/8, 12 is a multiple of 3 greater than three, 12/3 = 4 beats per measure, each of the 4 beats is subdivided into a group of 3 eighth notes.

Those subdivisions of three are grouped metrically, both visually in notation and by the metrical accent.

You have to do a quick bit of math on the top number to know if the time signature is simple or compound... and then you "read" them differently. But, for most practical cases 6/6, 9/8, 12/8 are the common compound meters.

Simple meter: 3/4, three beats, quarter notes get the beat, beats subdivide into two eighth notes...

Compound meter: 6/8, two beats, a dotted quarter note gets the beat, beats subdivide into three eighth notes...

Dance is probably the easiest way to distinguish the meters aurally. The minuet and waltz are simple meter 3/4, but the gigue is compound 6/8. The metrical feel of the two should be perfectly clear. If not, find some baroque dance suites with minuets and gigues, and listen to/play them.

Answer 7

Benrg's answer is really the correct one here, but I'd like to add a little historical perspective on why this convention exists. 3/4 meter generally represents 3 beats to a measure, where a quarter note is the primary beat. 6/8 generally represents 2 beats to a measure, where a dotted quarter is the primary beat.

To understand why this happened, it's useful to consider the older rhythmic notation of the renaissance, which ultimately led to our modern meters. We might consider four fundamental meters in renaissance music. (These were known historically as mensurations; see the table of "mensuration signs" in that link to see the historical notation I'm referring to in the following four meters.)

Let's assume that the whole note gets the beat (called a semibreve at the time, and still named that in many parts of the world). There was a desire to have groupings of two or three beats at a time, but also a desire to be able to divide up those beats into two or three subdivisions. Four possibilities emerge:

1. We could have a situation where there are 2 whole notes per major grouping (i.e., 2 beats), and each whole note is divided into 2 smaller notes (called minims, the ancestor of our modern "half note," but that latter name is misleading, as we shall soon see)
2. We could have 3 whole notes per major grouping, each divided into 2 minims
3. We could have 2 whole notes per major grouping, each divided into 3 minims (perhaps the minims here could be thought of as "third notes" instead of "half notes")
4. We could have 3 whole notes per major grouping, each divided into 3 minims

Effectively, options (1) and (3) have two beats per group ("measure" in our modern parlance), while (2) and (4) have three beats per measure. And options (1) and (2) have beats that are subdivided into two parts, while options (3) and (4) have beats that are subdivided into three parts.

Originally, all the "whole notes" looked alike, and you just had to know based on the meter when you were expected to count 2 minims per whole note vs. 3 minims per whole note. Similarly the next higher level above whole notes (breves) all looked alike too, and you just had to keep track of how many whole notes (semibreves) per breve.

But note something: triplet notation had not been invented, at least not as we use it today. So a "whole note" had to do double duty in a situation when it was divided into three parts: sometimes it was held for the full three count, and other times it was reduced to only two subdivisions, when followed by a minim. There were a huge amount of rules about when some notes were read one way or another way -- in different circumstances, the exact same note shape could stand for a value that was twice as long or twice as short or sometimes more exotic ratios.

By the 1500s, those triple subdivision meters were starting to die out in favor of a standard meter that looked like option (1) in the above list. In own modern system, we might call that meter 2/1, as it had two primary beats to a bar, with each beat divided into two parts. Rather than having notes that could look like they were sometimes longer duration and sometimes shorter, the option (1) became the standard used for meters even with groupings of three.

So, for options (3) and (4), each "minim" literally became a "half note," and the primary "beat" would now be notated as a dotted whole note, instead of the previous notation where the "whole note" (semibreve) could change its division based on context.

This was seen as a great simplification, but it led to a problem: how to indicate "compound meter" like in (3) and (4) above. (1) and (2) could be notated as 2/1 and 3/1, but in option (3) there are only two beats and the whole note doesn't have the beat.

The compromise that was eventually adopted was to go down one level in duration and use that for the denominator of time signatures, assuming that the 6 or 9 (and later 12) in the numerator would be assumed to represent a threefold division of the primary beat.

Hence, our four options could be represented using modern meter signatures as: 2/1, 3/1, 6/2, and 9/2.

Over the time that this was becoming standardized, however, there developed a preference for "black note" notation with shorter durational values. So these four options eventually became standardized at a quarter of their original length, i.e., 2/4, 3/4, 6/8, and 9/8. They represent the same concept of beat groupings and subdivisions as the four situations listed above, just now with a quarter or dotted quarter getting the primary "beat."

From a historical perspective, 6/8 is a "compound meter" because it was the simplest notation possible to capture the fact that it was distinct from 3/4 because the beat fell on the dotted quarter. If there were a way of representing 2/♩., that could be used instead as that was literally the situation that 6/8 was chosen to represent. Instead of using 2/♩., however, they chose a mathematical shorthand that started to generate confusion once the old system of meters died out and people forgot that 6/8 didn't mean "six beats per bar," but rather only two.