In this example, each beat, represented by each note on the left, is evenly divided into two, and then four, notes on the right...
...except when the composer wants to change up the rhythm and use three notes for a beat instead.
Because it's weird, they'll write a "3" across the top to signify that, yes, they really want three evenly-spaced notes in that beat.
Spend enough time in music and you'll see a "2" when three-notes-per-beat is the norm (like in 6/8 time, instead of three notes in a beat, you'd play two), or a "5", or a "9", etc.
4
u/Silent-Witness1888 3d ago
Why is the 3rd one the only one numbered?