In this post we will talk about measures (or bars) and time signature. I recommend that you first read the previous post, where we discussed basic rhythm figures, such as quarter notes, eighth notes, and so forth, right here.

Feel free to read (and re-read) the paragraphs of this post in any order, since it can be a bit hard to fully understand it.

Is time signature a real thing?

One of the main characteristic of a song, or any piece of music, is the time signature. The time signature defines, in a way, how we group notes. So yes, it does exist, with or without notation.

Grouping by 3

Listen to this excerpt of the Waltz in D♭ Major, by Frédéric François Chopin (1810 - 1849) :
(a waltz is a famous dance usually based on a 3 beats patterns)

You can listen to the entire piece here

From measure 5 (we'll talk about what is a measure right after that) to the end, you can strongly feel the 1-2-3 , 1-2-3 pattern.

Grouping by 4

In this score I've written a few bars of a common left hand piano boogie woogie pattern.

Can you feel the 1-2-3-4,1-2-3-4?

Other grouping ?

The most usual way to group notes is grouping by 4. As an example, most nowadays western pop tunes are written in 4.
But there is a LOT of wonderful music, from ALL PARTS of the world, written (or just played and sung) with completely different groupings. (in 5, in 6, in 7, etc)

Why do we need measures ?

This feeling of "groups of notes" is transcribed on the score.

Why do we want to group notes when writing ? We could just simply write notes one after the other, right ?
Ofcoursewecould,butyoucouldalsowritewithoutblankspacesbetweenwords!! Made my point?

So, to be clear :

Why do we have to group notes? (when writing)

Because it's way easier to read that way.

How do we groupe notes?

We group notes using measures

What amount of note will a measure contain ?

It is stated by the time signature.

How do I choose to group notes by 3, or by 4 ?

When writing music, we want the score we write to reflect the musical idea we have in mind. So if you are writing a traditional waltz, you should choose to group notes by 3. If you choose to write a traditional rock, you should choose to group notes by 4.

As a rule, the time signature you choose when writing music should correspond to the musical idea you have in mind.

Definition of time signature and measure.

We get to the point !

Time signature

The time signature is a set of two numbers. It specifies how many beats are to be contained in each measure (upper number) and which note value correspond to one beat (lower number).

For now, the lower number will always be 4, since we work with the convention "one beat = one quarter note". (Sometimes, the lower number is 8, meaning "one beat = on eight note") . So let's forget the lower number.

The upper number simply correspond to the kind of grouping. If it is a grouping by 3, like in a waltz, it's 3
If it is a grouping by 4, like in a waltz, it's 4.

The time signature is written at the beginning of a piece. Have another look at those excerpts, and see how the time signature is written.

In this piece, the time signature is 3 *(remember, for now we don't care about the lower number)*. It means that we make groups (ie measures) of 3 beats.

In this piece, the time signature is 4 *(remember, for now we don't care about the lower number)*. It means that we make groups (ie measures) of 4 beats.
Measure (or bar)

A measure (or bar) is a group of note, that has a length specified by the time signature.
The boundaries of the measure are indicated by vertical bar lines.

Here are 4 measures of music :

Again, have a look at that :

In this piece, each measure contains 3 beats : it can be 3 quarter notes, or 6 eighth notes, or any combination leading to 3 beats. In this piece, each measure contains 4 beats : it can be 8 eighth notes, or a whole note.

In this last score, I just put different combination of rhythm feeling 4 beats.