Regardless of which time signature is used, all beats of every measure
must always be occupied fully by some combination of either notes or
rests. The total value of all notes and rests in a measure may never be any
more or less than the number indicated by the time signature.
Ties
A tie is a curved line that connects two notes of the same pitch for the
purpose of combining their time value. Tied notes are treated the same as a
single note, with the second note held as an extension of the first. When
writing a tie, we should begin as close to the note head as possible, and
curve away from the stem of the note.
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Ties are used in three places:
1 ) Across a bar line: when the sound of a note sustains across a bar line,
a tie must be used to maintain the proper number of beats in each
measure.
2 ) Across the middle of a measure with an even number of beats: when
writing music, it is very important to keep the notes in the measure
visually organised. By using a tie to sustain a note over the middle of
the bar, rather than a single, longer note value, it is easier to see the
division of the measure and therefore easier to read the music.
3 ) Across the beats of a measure when writing sixteenth notes or rests
(or smaller values): for the same reason that the middle of the bar is
kept clear by use of the tie, each beat must visually stand alone when
there are small note values, in order to avoid confusion.
Although at first glance they appear to be the same, a tie should not be
confused with a slur, which is used to indicate the smooth phrasing of two
or more notes of different pitch.