How can you sing when your notes aren't round? How do you interpret a diamond-shaped note or a triangle?
This isn't avant-garde notation, but one that dates back a couple of centuries and is still alive today.
This particular hymn-singing tradition in the US comes from the southern states and is named after one of the early collections of these hymns, The Sacred Harp.
Music in The Sacred Harp hymnal as a couple of interesting characteristics: the melody is in the tenor voice and the notation is not the usual round note heads we may be familiar with.
This music is written in what are called 'shape notes,' where the different shapes of the note heads, square, round, triangular and diamond, indicate different pitches.
This makes it possible to read the music two ways. If you can read music, then you just read the notes. If you can't read music, then you read the shapes.