Friction idiophones is designation 13 in the Hornbostel-Sachs system of musical instrument classification.
These idiophones produce sound by being rubbed either against each other or by means of a non-sounding object.
Instruments of this type are not very common; possibly the best known examples are the musical saw and the nail violin.
It is essential to distinguish between rubbing and scraping.
The latter results in a series of beats cause by passing a stick over a notched surface; [whereas] friction is based on adhesion....On a higher level we find...the glass harmonica.131.1 Individual friction sticks.