Eli5: Music theory: Why do we have certain time signatures?


If signature does not denote tempo, why do we need signatures like 2/4 or 6/8? Why can’t the music just be written in 4/4 or 3/4?

In: 30

Signature doesn’t denote tempo, but it does denote what beats are emphasized.

You typically emphasize the first beat of a measure. If you’re just singing and playing guitar, you typically change chords on the first beat of a measure. So if it’s 4/4, you play one chord four times, then you change to a different chord. In 3/4 you’d change after 3.

Sometimes when writing sheet music, it’s easier to write in a different time signature to make the flow of reading it easier. For instance, writing in 7/8th for a piece would save having to constantly jump between 4/4 and 3/4 every other bar.

Or maybe the piece has an off-beat bridge between 2 sections that’s only half a bar long.

Or for piano, sheet music has both left and right hand on 2 different lines, maybe your right hand plays twice the speed as the left hand. Either way the performer needs them to be written at the same place on the page, otherwise you go cross-eyed trying to read it in 2 different areas!

You could write many pieces of music in just the simplest time signatures – but it is easier to use a more appropriate time signature!

For example, if you were writing a piece of swing music, with a rhythm like DA-da-da DA-da-da DA-da-da DA-da-da, then you could write that as 12/8 (12 half-beat notes in the bar), or as triplets in 4/4 (4 single-beat notes) or 8/8 (8 half-beat notes in the bar). Since writing every single thing in triplets adds a lot of extra hassle for no good reason, you might decide to use 12/8 time so that triplets are not required and everyone’s life is easier.

Furthermore, when writing with words to achieve a particular effect, you might make some of your sentences longer and some of them shorter, since these different lengths of phrases might achieve slightly different effects. It’s the same with music. You could write everything in 1/4 or 2/4 or 4/4 or even 8/4, and these might all end up being more or less the same but just with the barlines in different places – but since we might emphasise the first beat of the bar a little more than the rest, each of these choices might end up sounding a little different when played by a musician capable of putting some expression into their performance.

If you can’t quite visualise that, then imagine having a conversation with a friend: what if you spoke in sentences that were longer than usual? What if you made every sentence short and punchy? They would sound differently when you performed them aloud, even if you still used the same number of words in total to complete your side of the conversation.

Signature isn’t for tempo, signature is to tell you;

1) What kind of note denotes a single “beat,” and;

2) How many beats you have per measure, as well as (generally) what beats should be emphasized.

>2/4 or 6/8? Why can’t the music just be written in 4/4 or 3/4?

Because theses actually mean different things, even if they’re “mathematically” equivalent at first glance.

2/4 time would denote that the first beat gets emphasis. 4/4 time denotes that the 1st and 3rd beats get emphasis, but that the 3rd gets less emphasis.

3/4 and 6/8 are also different; 3/4 time denotes that the 1st beat gets emphasis, whereas 6/8 time denotes that the 1st and 4th beats get emphasis, with the 4th beat having less emphasis.

So summarize others’ ideas.

There are twomain reasons.

1) When writing sheet music, there will be ways to write it to make it easier or harder to read for musicians. And sometimes the way you write the music might subconsciously affect how the musician plays it. (This isn’t a huge reason, and it’s related to No. 2, but it’s there.)

2) The most important reason, simpler or more complex time signatures allows a composer to rely more or less on primary and secondary beats of emphasis.

Say the word “Summertime.” The first syallable is stressed the most, the “um” is unstressed, but the “time” has a little bit more stress than the “um” but less than the “sum”.

Compare that with “violet”. “Vi” is stressed, but “ol-let” are more or less the same.

Similarly, in music, the first beat of a measure is stressed. If it’s just 2/4 or 3/4, then you have the first beat stressed, and the second (or second and third) equally unstressed. Listen to the [Stars and Stripes Forever](https://youtu.be/a-7XWhyvIpE?t=57) march, or a Strauss waltz. Or something like ‘[Ice Cream](https://www.youtube.com/watch?v=8K0qCCfeMAE)” by Sarah McLaughlin.

But when you get into 4/4, or 6/4 (or more commonly, 6/8), you get a measure with a primary stress on 1, and a secondary stress on 3 (or, in 6/8, a primary stress on 1, and a secondary stress on 4). Compare “Ice Cream” with “[Gravity](https://www.youtube.com/watch?v=7VBex8zbDRs)” by John Mayer. It feels different, right? In fact, what John Mayer is doing is hitting the back beat in 6/8–emphasizing the less stressed big beats in a measure is something that started with blues and is a staple of rock music. But you can hear that bass drum hits on 1, and the snare drum hits hard on 4. That’s different from just a 1-2-3- measure. Now listen to Ice Cream. Where’s the snare drum there? It’s on beat 3–that’s the accented off-beat. Now, *could* “Gravity” be written in 3/4, with some weird accents written in to indicate that beat 1 of the second measure should feel different from beat 1 of the first measure? Sure. But the inclusion of the barline would send the wrong signal to a musician.

Similary, compare Stars & Stripes with something like “[We’re not gonna take it](https://youtu.be/V9AbeALNVkk?t=166)” Stars and stripes is definitely in 2. There’s and equal, strong emphasis on every second beat. Twisted Sister’s song has a strong 1 (quiet 2) 3 (but not as strong as 1), (really quiet 4). “Gon'” is not as strong as “We’re” but it’s stronger than “not” or “take”.

(Of course, there are also compound time signatures, like 5/4, or 7/8, but they too play around with primary and secondary emphasis. But that’s a different question.)