String instruments, including guitars, violins, cellos and double basses must follow the same principle as woodwinds and brass instruments, and that is that the longer a sound wave, the lower the pitch. The difference is that string instruments make soundwaves mechanically instead of through the use of exhaled air.

When a string vibrates, it creates a sound wave. The longer that string is, the longer the sound wave is and the shorter that string is the higher the sound wave. The width of the string also plays a part: thinner is higher, while thicker is lower. On a violin, for example, there are four strings each tuned to a different note based on their width. This gives a violin four basic notes that can be played by either plucking or drawing a bow across the string.

By changing the length of string that can vibrate, the violinist can change the pitch of the note played. This is why violinists hold the strings with their other hand. The same principle works with guitars, violas, bass guitars, etc.

Harps and pianos take the easy way out. They just have a string for every note.