It is quite clear that a letter like the hard c is sharp in a way that z is not. There are all sorts of relations of sharpnesses and widenesses, and keennesses and softnesses or envelopingnesses in a poem. And take perhaps along with the hard sound of c the keenest letter in the language, n . N does happen to be the letter used when you want to deny something. All the letters are presentations of keenness or softness in one way or another.
Swiftness is associated with keenness.
We have also the visual effect breaking.