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I know the general idea behind time signatures:
6:8 is 6 eighth notes in a bar, 3:4 is 3 quarter notes in a bar.
I also understand that in situations like these, while the two technically equal each other mathematically, they have different stressed beats.
However, I'm confused as to how these stressed beats are found. In other words, what if I was given a time signature like 12:8? 4:2?
I've heard the rule that in x:8 time, there are always groups of three, but what else do you have to go by?
The simplest way to derive which beats are stressed in a given time signature is to look at how the notes are beamed or agogically accented.
For example, 6/8 is typically divided in a 3+3/8 grouping (as noted in other answers), however, it could also be grouped 2+4/8, 1+5/8, 4+2/8, and 5+1/8 for example. Each of these different divisions stresses a different beat in the measure, and must be grouped accordingly. Through notation, you can distinguish groupings through how notes are beamed.
This grouping tactic is even more pronounced in asymmetric meters, like 5/8 and 7/8. In these groupings the stresses must become lop-sided, so a 5/8 typically turns into a 2+3/8 or a 3+2/8.
If the time signature doesn't allow for beamed groupings (such as 7/4), then it is important to look at phrasing, and if possible, agogic accents. If in 7/4 I saw two half-notes followed by three quarter-notes, I would know that it had a 4+3/7 grouping.
Alternatively, look at phrase or slur markings to help you distinguish which beats begin / end phrases. Look at other instruments (if available) in the score to see if another part can clarify the confusing grouping. Listen to other recordings and see how others have phrased it. Lastly, play through it and figure out which phrasing sounds good to you.
As so often in music, we're going to struggle to pin down a Theory of Everything.
No, I'm afraid a bottom number of 8 doesn't always imply Compound Time (3-groups) and one of 4 doesn't always imply Simple Time (2-groups). 6/4 is Compound Duple - two dotted half beats. (We use 3/2 for Simple Triple - three half-note beats.) 2/8 and 4/8 are alternatives to 2/4 and 4/4. And here's one of Theory's dirty little secrets - although the textbooks don't mention it much 3/4 and 3/8 are often Compound Single - a one-in -the-bar fast waltz or a jig.
Today's composers will often choose 8/8 (rather than 4/4) when the music falls into irregular rhythmic groups, their alternative might have been to notate something like 2/8 + 3/8 + 3/8.
But, in Theory 101, you'll be pretty safe if you consider a time signature with top number 6, 8 or 12 to be Compound (3-groups), 2, 4 or 8 to be Simple (2-groups). Top number 3 is tricky. The textbook answer is probably Simple Triple - three 2-groups. But in real life it could well be Compound Single - one 3-group.
Some of the above answers have done a great job of breaking this questions down, so I'm going to keep it simple. The way I think about time signatures is about feeling.
For instance, if I'm playing in 3/4, then there are three pulses per bar made up of a quarter note for each pulse. If I'm playing in 6/8, then its two pulses per bar with each pulse made up of 3 8th notes. In 12/8, 4 pulses per bar made up of 3 8th notes per pulse.
To expand that, 5/8 is (depending on the accents) 2 pulses per par, one pulse containing 2 8th notes and one containing three 8th notes.
To extend that even further (if you are crazy like me): 19/16: I would feel it in 3 groups: 1 group of 7/16 - so 3 pulses, 2 pulses of 2 16th notes, 1 of 3 16th notes, 1 group of 5/16 - 2 pulses, one pulse of 2 16th notes and one pulse of 3 16th notes, and then another group of 7/16.
A lot of this depends on the piece of music of course, as a melody might not be easily subdivided into pulses, but again, these are common ones, and you will be able to feel a large number of tunes with these sort of ideas.
My suggestion is to listen to the music, break it up into recognisable pulses, then work out how the melody travels over them, and then go from there.
Good luck!
In 6/8, the unit of beat is the dotted quarter note, so you have two beats that are divided into triplets - that's to say that you would count the beats something like "One-two-three, two-two-three." It's duple (or two beat) time, but the beats themselves are divided by three. In 3/4 time, there are three quarter note beats to the bar, divided into two eighths apiece.
In 12/8 time, the unit of beat is also a dotted quarter note, so you have something with a beat pattern like 4/4 time (four beats to the bar, with a stress on the first beat and a lesser stress on the third), but each beat is divided into triplet eighths rather than duplets.
In general, if the number of beats (the top of the time signature) is divisible by three more than once (which means that time signatures like 3/4 and 3/8 don't apply here), you have a time signature where the unit of beat is equal to the three of the note values specified by the bottom of the time signature. Thus you can have a time signature of 15/16, which means you have 5 beats, each with a note value of a dotted eighth, or 6/4, which means two beats, each with a note value of a dotted half note.
NO! 6/8 is not six groups of quavers it is compound duple time with each bar consisting of a dotted crotchet. If you group six quavers together it implies 3/4 time.
12:8? Here you have compound Quadruple time with each beat consisting of a dotted crotchet
4:2? Here you have four groups of minims
Now let us think critically of what a time signature is. Time signatures at is very root try to explain to you the amount of time or notes that the bars of the music should have.
There is two numbers but what do they mean I can hear you ask? Well the first number tells us how many groups of notes we have and the second tells us what each group consists of.
So let us take 2/4 time as an example. The first number tells us the number of groups ie 2 and the second number tells us that the groups consist of crotchets ( 2 = minims 4 = crotchets 8 = quavers)
But wait there is more. There is a second type of Time signatures. They are called compound time signatures. No all these do is add dots to our groups but for the most part the same effect applies.
6/8 is compound duple time with groups of dotted crotchets. 9/8 is compound triple time with beats of dotted crotchets and so on and so forth.
Hope that helped.
