The values indicate beat counts and subdivision counts, and are chosen typically as a coordinative and interpretive aid.
To illustrate: A waltz (i.e. – The Blue Danube) is in 3/4. A shuffle (i.e. – Just A Gigolo) can typically be in 6/8…but can also be in 4/4 and notated in triplets. The choice the composer makes is often based on the ‘feel’ they are trying to illustrate.
(Always remember that music is full of rules that you have to follow…until you get good enough to understand which ones can be *bent* and which ones can be *broken*).
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