Melody
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A melody in music is a tune. A melody is a line of notes of different pitches (How high or low a note is) which are played one after another. Together they make a tune in the same way that a group of words make a sentence. Some people like to sing melodies. Melodies have rhythm (the length of the notes).
In music, 'melody' contrasts with 'harmony'. Harmony means notes which are played at the same time, like chords. Composers often think of a melody and then add harmony to it.
The adjective of melody is melodic. There is also the adjective melodious which means: 'a sweet-sounding melody'.
Music that does not have an obvious melody may be hard to understand (unless it is focused on rhythm).