A musical saw, also called a singing saw, is a hand saw used as a musical instrument. Capable of continuous glissando (portamento), the sound creates an ethereal tone, very similar to the theremin. The musical saw is classified as a plaque friction idiophone with direct friction (132.22) under the Hornbostel-Sachs system of musical instrument classification, and as a metal sheet played by friction (151) under the revision of the Hornbostel-Sachs classification by the MIMO Consortium.

An idiophone is any musical instrument that creates sound primarily by the vibration of the instrument itself, without the use of air flow, strings (chordophones), membranes (membranophones) or electricity (electrophones). It is the first of the four main divisions in the original Hornbostel–Sach system of musical instrument classification. The early classification of Victor-Charles Mahillon called this group of instruments autophones. The most common are struck idiophones, or concussion idiophones, which are made to vibrate by being struck, either directly with a stick or hand or indirectly, with scraping or shaking motions. Various types of bells fall into both categories. A common plucked idiophone is the Jew's harp.