The Melodic Line
When composers first put pen to paper, they translate a melody in their mind onto a lined piece of paper by writing notes in a particular key on the lines or spaces between the lines. This is called musical notation. Looking at the notes on the paper, it is possible to sing or play the melody (tune) from the musical notation and to change the notes on the paper until the melody reflects what is in the mind of a composer.
In its most simplistic sense, this is the starting point for all music, whether instrumental or vocal. We almost never hear music that has only a melodic line, unless we are learning to sing a song. Normally, what we hear is an orchestrated piece of music. An orchestration is a translation of that one melodic line into a “multi-line” piece of music.