Music is divided up either by 2’s or by 3’s, stated as “duples” or “triplets” respectively.
So if we were to count 1 – 2 – 3 – 4, we can count between the numbers using “and” like this – 1 & 2 & 3 & 4 &. If the numbers were quarter (1/4) notes before, by adding the ands we can count eighth (1/8) notes. And we can subdivide further to sixteenth (1/16) notes by adding something between the “ands”. We count this as 1-e-&-a-2-e-&-a-3-e-&-a-4-e-&-a.
Triplets subdivide the 1-2-3-4 into a “triplet 8th note” by counting 1-&-a-2-&-a-3-&-a-4-&-a.
Laid out graphically this looks like:
All of this follows logically.
Where it gets interesting is with an eighth note swing feel. Technically it’s still a duple, and this is how you would count it, but if you swing the count, it can begin to take on the feel of a triplet.
This is where you get into the “groove”, the space between the notes. Defining how much swing, or how things feel – the human element rarely follows a perfect duple.
The “groove” is one of those things you pretty much need to stick with for an entire song, and everyone needs to agree on – because if you look at the jumble of the grid below, you can see a pretty bad train wreck if you tried to mix the triplet or swing feel with the straight duple feel. The same things happens if you mix other grooves.
One groove per song keeps everything locked into place and feeling tight.