Which element can a composer use to lengthen the duration of tone

The element is called "Dotted Note"

Explanation:

The dotted note is used to increase the duration of the basic note by half of it's original value. Dotted note consist of a small dot that is written after it.

It is basically use to represent the change in the duration of the note.

We have the following types of dotted note

Dotted whole not, for 6 beats

Dotted half note for 3 beats and so on.

The type of dotted note to use depends on the duration of tone you desire.

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