All right, here’s what I think I know:
6/8 time is 6 eighth-notes/quavers per measure.
It has six counts per measure.
It has 2 “beats” per measure.
Now, this confuses me. If a song is “Allegro,” am I counting quickly (i.e. six quick counts per measure) or “beating” quickly (i.e. 2 quick beats per measure, each one equally a dotted quarter-note)?
Please correct me if my premises are wrong.
Typically when music is written in 6/8 it is felt with two beats. The way I seem to interpret it in my head is that it is comparable to a 2/4 measure with a quarter note triplet subbeat.
You know how when you learn 4/4, they teach you that there’s two “big” beats and two “small” beats, and the second big beat is slightly smaller? You know:
Da-da-Da-da.
That’s primarily what differentiates it from 2/4, which has the same rhythmic math, but has a single “big beat” at the beginning of each measure.
Da-da.
There’s an analogous relationship between 3/4 and 6/8.
3/4: Da-da-da.
6/8: Da-da-da Da-da-da.
ONEtwothree TWOtwothree / ONEtwothree TWOtwothree
The tricky concepts are
a) that the dotted quarter note gets the beat. It’s more intuitive for a non-fractional note (quarter, half) to get the beat.
b) that the beat note isn’t on the bottom of the time signature. It usually is.
Think of it as a fraction in lowest terms. You couldn’t write a time signature of 2 over dotted quarter, so you express it as 6/8.
So, how does 6/8 differ from 3/4 except for having twice as many notes per measure? Couldn’t you just write it in 3/4 and have twice as many measures?
An interesting song to listen to where you can hear the difference is in the musical Man of La Mancha. The song is called Dulcinea.
The measures alternate between 6/8 and 3/4 time. Throughout the whole song.
I can still remember my junior high choir director counting it out.
“ONE two three FOUR five six / ONE and TWO and THREE and / ONE two three FOUR five six / ONE and TWO and THREE and”…etc.
If you can find a version, listen to it, and appreciate it in a whole new light.
The fractions don’t reduce in time signatures, psychobunny - you’d have to write it in 3/8 to use twice as many measures; this would (or ought to, anyway) change the way it sounds by having the downbeat every three instead of six.
If you doubled all the note values, you’d have it in 6/4. This is what’s usually used for three sets of double-beats, to get back to the OP, or possibly one set of six.
So the basic difference between /8 and /4 is that /8 is usually for grouping in threes, and /4 for grouping in twos.
The other common example of this is America from West Side Story:
I like to BE in A- | Me-ri-ca | O-K by ME in A- | Me-ri-ca | …
And 633 Squadron, intentionally (a sort of musical pun on the name, as it’s going 6-3, 6-3, 6-3… not actually 6-3-3 which would be harder to manage).
The Mexican Hat Dance is another instance of fast 6/8 time - you’d beat that in two as it’s far too fast to beat the individual quavers.
6/8 is a compound meter: there are two beats to a bar, a strong one and a weak one (One,two|One,two|One,two)… Each beat is further divided into three (One da da, two da da|One da da, two da da|One da da,two da da)
So, you could translate it into 3/4 or 3/8, but the pulse should be in two’s. I think 3/4 should be played with a bit of a lilt, definitely a pulse in 3’s.
Ah hah! A hint. So I can tap out 6/8 in either 6s or 2s, depending on the song? This would let me adjust the tempo a little more nicely, to boot, eh?
Re-reading I realized I read the question wrong. Yes, sometimes the tempo indication means 2 counts or 6, depending on the song. Often the style of the song makes it clear, or there is further indication (like dotted eighth note = 70 or something like that). I’ve even seen multiple indications (for when played in two or six) but I think that was only in practice material.
Sometimes even full measures don’t get a downbeat. The scherzo in Beethoven’s Ninth is in 3/4, but at such a clip (molto vivace) that it’s three or four measures to a downbeat.
Yes, that’s spot on.
I prefer to reverse K364’s explanation, for the terminology to make sense: a compound metre takes fast quaver beats, and groups (i.e. compounds!) them into larger beats. When teaching it, I demonstrate it visually, relating it to how I’d conduct 3/4, and 4/4. Then 6/8, conducting six beats, but then it needs to go faster, so it becomes ridiculous. So I conduct the two large beats instead.
I presume you mean dotted quarter? Unless you’re talking about 6/16 or similar 
Yes, sometimes the tempo is slow enough that the six beats are all felt, however, there’s still a sense of grouping them into threes in terms of the phrasing, of the harmony (perhaps one chord per half-bar) and so on. If it were a slow 3/4 you’d also feel six sub-beats, but with the different grouping (into twos) evident.
A more extreme case occurs in the final moments of Elgar’s Enigma Variations, at the very last reappearance of the theme. He puts brackets across three bars indicatings that this is now to be a single conducted beat - IIRC they’re three bars of 2/2.
I use Band In A Box music accompaniment software. This only allows a choice of 3/4 or 4/4, but sometimes I want to score something in 6/8. So I choose a 4/4 style with a triplet feel, and then run the output through a midi tool that tweaks the note durations to get everything right.
Works a treat.
Si
That’s pretty much how I learned 6/8: like 2/4 with triplets.
A lot of John Phillip Sousa music is in 6/8.
Amazingly enough, his music was considered ‘dance’ music of the day. You did the new ‘two-step’ to his stuff instead of waltzing all the time.
6/8, 9/8, 12/8 are very similar to 2/4, 3/4, and 4/4 respectivly with triplets.