Music can be described as sounds and silences in time:
Rhythm is about when to play and how long to play for.
Time in music is a many faceted gem. The control of time allows us to sustain sounds for specific amounts of time or to sustain silences in the same way.
As in comedy, so it is in music, timing is everything.
Everything in music is about controlling time.
When we control the time between pulses we are controlling pitch and frequency.
When we control the time of individual notes and between individual notes on an instrument, we are controlling rhythm
Sounds come in all varieties, long and short, loud and soft, high and low. The most fundamental aspect of a sound is how long it lasts: what amount of time passes by while it is sounding?
We actually have a range of notes that we can use, each telling us how long a note should sound for. However, instruments (and singers) don’t play all the time; they need time to breathe or the music needs to pause to increase its effect and meaning, so we also have a range of rests indicating how long an instrument should be silent.
The relationship between notes is relative. The semi-breve 
is the longest single note we usually see today. Each of the other notes in the diagram below are half as short again:
has a value of half of the semi-breve, so it is 1/2 of the semi-breve
has a value of half of the minim, so it is 1/4 of the semi-breve
has a value of half of the crotchet. so it is 1/8 of the semi-breve
is one half of the quaver, so it is 1/16 of the semi-breve
has a value of half of the semi-quaver. so it is 1/32 of the semi-breve
is one half of the demi-semi-quaver, so it is 1/64 of the semi-breveIt is this relationship between each of the notes that the American note names are based on:
We can add a dot to every note, extending its value by exactly half as much again.
In both simple and compound meter we can deny the usual division of the beat.
By dividing Simple meter beats into three (triplets):
where the 
, instead of being divided like this 
is divided like this 
.
By dividing Compound meter beats into two (duplets) or four (quadruplets):
where the 
instead of being divided like this 
is divided like this 
or this 
or 
.
Other divisions are possible and are left as advanced investigations at this time.
Music is sometimes described as the regular (or irregular) pattern of sound and silence having identifiable pitch and structure. Most music that we can think of fits this description.
When we combine notes together, we can make larger units of rhythmic structure. This larger structure usually has a regular underlying pulse. This pulse is usually one of the larger units of durations such as the crotchet, , or the minim, . It is sometimes represented by the quaver, . but rarely by the semi-quaver, .
The regular pulse will also indicate how many occurrences of the pulse duration are grouped together. This is done through the accenting of the first note within the grouping, as well as other minor accents throughout. These groupings are called bars and we introduce a vertical line between groupings to indicate the start of each new grouping or bar.
A time signature tells us two things about the regular pulse:
The length of the beat allows us to think in simple terms about how we use rhythmic notation to display the rhythms that occur. Some modern notation systems just use grid paper and mark off the time elapsed in ms. Others write the number of beats on top and show the actual note type on the bottom. In any case we are usually concerned about grouping beats together - I don't know of many musicians or composers who don't work within some kind of rhythmic framework.
When working with other musicians it is important that everyone knows what the beat value is, less so for computer based work. However, my experience of DAWs as they have developed over the last 3 decades is that they are inherently based on rhythmic division of a bar, so beat value becomes important when working within them.
So a time signature of 2/4 tells us:
) is the beat value
A key point to remember is that these notes don’t stand for any particular amount of time - the crotchet doesn’t last for 1 second or ½ second or any thing else you can think of. It stands for a relative amount of time based on the tempo (or speed) that a piece is played at.
Tempo is the speed at which music goes. We usually define the tempo in one of two ways:
= 120, says that there are 120 crotchets (quarter notes) per minute. MM = 120 says that there are 120 minims (half notes) per minute (it also implies that there are 240 crotchets (quarter notes) per minute). The MM stands for Maelzel’s Metronome, after the man who created the metronome in the late 18th century, although today it could as easily mean Metronome Marking.The MM marking is sometimes stated as BPM
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