A violinist can place their finger very lightly on a string so that it divides the string into half. They will hear a harmonic (the note an octave higher than the open string). By placing their fingers in other places they can get more harmonics, e.g. by touching the string a quarter of the way down they gets the next harmonic. “Artificial harmonics” can be played by stopping a string with the finger in the usual way (so that the string is now shorter) and placing the little finger farther up the string to get a harmonic of the stopped note. Artificial harmonics are written with diamond-shaped note heads. They are very hard to play well.