Quite often, the beginning of the song will start with what
is called an incomplete measure.
An incomplete measure where there are not enough notes or rests in
the measure to equal the top number in the time signature.
Whatever number of beats are missing in the
first measure will be "picked-up" usually at the end of the song
in the last measure - which will also be an incomplete measure.
Therefore, the number of beats in the first
measure plus
the number of beats in the last measure will equal 1 complete
measure.
