A string instrument is a musical instrument in which sound is produced by the vibration of one or more strings. Organology classifies them in the category of cordophones.

The history of stringed instruments goes back thousands of years. The former probably only had one string, like the musical bow. Harp players were known as early as ancient Egypt. In the Middle Ages, minstrels accompanied themselves on the lute, etc.

Bowed string instruments

These are the violins and similar instruments of the European Symphony Orchestra, and many popular and scholarly musical instruments around the world, such as the Chinese erhu.

In most cases, the strings are rubbed with a bow. The lightly adhesive surface moves the rope, until the return force to its resting position exceeds the limit of adhesion. The rope then returns to its rest position, passes it and comes back in the other direction; at a certain point, the speed of the string relative to the bow is zero, and it sticks again. This process produces a vibration, which has the peculiarity of having a fundamental frequency slightly different from that of the string vibrating without excitation. This difference depends on other parameters, such as the nature of the coating on the bow, in general, a resin called rosin, the contact force, the speed of movement, controlled by the player, as the main parameter, the length of the string, which the musician adjusts, in the case of the violin, by pressing one or more strings on the neck. The vibrations are transmitted through the bridge to the sound box.

The open string, the length and tension of which are fixed, varies a little in fundamental frequency with the downforce and the speed of the bow, at the same time as the sound volume changes. The bowed string instruments of Western orchestral music, of the violin family, do not have frets. This is what allows the instrumentalist to play just the different notes, while varying the three causes which affect the power and the fundamental frequency of the vibrations which are transmitted by the bridge to the resonance chamber.

The hurdy-gurdy rubs the strings with the edge of a disc driven by a crank. The construction and adjustment of the instrument determine the downforce on the string.